The Unicorn and the Wasp (TV story)

The Unicorn and the Wasp was the seventh episode of the fourth series of Doctor Who. It saw Doctor Who's take on the disappearance of famous author Agatha Christie in 1926.

Synopsis
In 1926, Agatha Christie mysteriously disappears, only to be found ten days later at Harrogate Hotel with no memory of what happened to her. What could have been the cause? Was it a nervous breakdown? Was it a cry for help? Or did it involve a giant alien wasp and a mysterious stranger known only as the Doctor?

Plot
The TARDIS materialises outside a country estate, hidden by trees; the Tenth Doctor and Donna Noble step out. The Doctor smells the air and tells Donna that they have landed in the 1920s. Donna wonders if the Doctor really could tell the year by smell, but then points out that the vintage car coming up the driveway may have given it away. They both hide.

The car's driver, Professor Peach, parks outside the house. He is promptly greeted by his old friend, the butler Greeves, just as the local reverend, Arnold Golightly, arrives on his bicycle. As the servants take their luggage, they exchange a few words. Peach decides to go to the library to do some research on his own. Golightly tells Professor Peach that they're at a party, he should try to relax; constantly working will be the death of him.

Watching from the side of the house, Donna tells the Doctor to forget about planet Zog — a party in the 1920s is much more fun. The Doctor points out that they're not invited, before flashing his psychic paper — oh yes they are! They head back to the TARDIS so Donna can find a dress from the 1920s to wear.

In the library, Professor Peach looks at one document and realises that he was right about something kept secret all these years. Suddenly, a shadowy person comes in, and Peach quickly hides his papers, telling the person that he was just doing some mundane research. He then asks the mysterious person what he is doing with a lead pipe. The mysterious person's eyesight becomes purple as Peach mumbles, "That's impossible!". The mysterious person, revealed to be a giant wasp, suddenly swings the piece of pipe at Peach's head as the Professor screams...

Back outside, the Doctor is impatiently waiting outside the TARDIS for Donna to finish changing into proper attire, reminding her that they'll be late for cocktails. Donna comes out in a period dress and with her hair done up and asks, "What do you think? Flapper or slapper?" The Doctor smiles and responds, "Flapper. You look lovely." They head for the front lawn. They are greeted by Lady Clemency Eddison, who wonders who they are. The Doctor uses the psychic paper to fake an invite and comes up with a story about meeting her at an ambassador's reception. Lady Eddison excuses herself as she is being cautious because the Unicorn is about. The Doctor, at first, mistakes this for an actual unicorn before Lady Eddison clears up the confusion by explaining that "the Unicorn" is a notorious jewel thief who is on the loose and has just struck again.

Next, they meet Lady Eddison's wheelchair-bound husband, Colonel Hugh Curbishley, and their son Roger, who starts to hit on Donna. Donna is confused why Lady Eddison has a different surname and the Doctor explains that the Eddison title descends through Lady Eddison, not the Colonel; one day Roger will be a lord. Roger secretly flirts with Davenport, the male servant; Donna and the Doctor easily pick up on the homosexual relationship between them and whisper quips to each other. (Donna says, "All the decent men are on the other bus," to which the Doctor adds, "Or Time Lords.")

Next to arrive is Reverend Golightly, whom Lady Eddison congratulates on the apprehension of two boys who tried robbing his church last Thursday night. Following him is the socialite Robina Redmond. Last to come is British mystery writer, Agatha Christie. The Doctor is ecstatic about meeting her, as she is another of his favourite authors. He admits that he was only surprised once by her books, but "It was a good once!". Still a young writer at this point, she has recently published her sixth novel. The Doctor and Donna are both impressed when she calmly notices that they're not married, as Donna has no wedding ring. Agatha is, however, unwilling to talk about her husband, who will not be joining them. Seeing that they are a person short, Lady Eddison tells Miss Chandrakala to go into the house and find Professor Peach. Golightly mentions that Peach said he was going to the library.

The Doctor acquires a newspaper from Hugh's chair and reads it. He immediately realises that something is wrong when he shows Donna the date — 8 December 1926, the day Agatha Christie disappeared. He explains that Agatha has just discovered that her husband is having an affair. Being British, these people would normally just carry on (with stiff upper lips), but this time, that won't happen. Tomorrow morning, her car will be found abandoned by the side of a lake; ten days later, Agatha will turn up in a hotel in Harrogate with no memory of what happened. She never spoke about what happened to the day she died. A surprised Donna then asks the Doctor, "Then it's about to happen..."; he finishes "...right here, right now." Their train of thought is interrupted when Miss Chandrakala comes running back, yelling frantically that the Professor has been murdered in the library!

The Doctor and Donna race to the library to find the Professor's body, followed by Agatha. The Doctor determines that blunt force trauma was the likely cause of death, and notes that the watch broke as the victim fell — pinpointing the exact time of death to 4:15 PM. Agatha discreetly takes a piece of paper from the fireplace, but the Doctor notices her reflection in the bookcase. Donna softly asks the Doctor if he noticed the professor's murder is like the board game "Cluedo". The others arrive on the scene, and decide to call the police. However, the Doctor uses his psychic paper to identify himself as Chief Inspector Smith of Scotland Yard (aka "the Doctor") and Donna as "the plucky young girl who helps me out". Donna takes issue with this description, but the Doctor explains that there are no policewomen in 1926.

While Agatha keeps the others in the sitting room until he's ready to question them, Donna asks the Doctor why they aren't calling the real police. He explains that he has found "morphic residue" — a by-product of shapeshifting — on the floor. This means that one of the others is an alien in human form. Donna then says the situation is weird; Agatha Christie wasn't literally surrounded by murder. She compares it to Charles Dickens being surrounded by ghosts at Christmas. The Doctor hints he actually experienced that, as Donna then asks if they could "drive cross country and find Enid Blyton having tea with Noddy", before asking if Noddy is real. The Doctor confirms he isn't, then rushes off.

While the Doctor tastes the residue to determine what left it, they walk past the sitting room. Donna then asks if it's like Murder on the Orient Express, where everyone did it. Agatha overhears this and finds the idea brilliant. Donna tries to tell her it's one of her best books, but the Doctor quietly hints to her that it hasn't actually been written yet. The Doctor explains he and Agatha will question the suspects, handing Donna a magnifying glass to search the rooms upstairs for clues (and, he whispers, more residue). The Doctor expresses joy at being able to solve a murder mystery with Agatha. She reluctantly agrees to work with him, unhappy at the Doctor's casual attitude toward the murder.

During the interviews, the guests recount their stories of what they were doing at 4:15 PM. Each is revealed to be hiding something, except for Reverend Golightly (who claims to have been unpacking in his room). Lady Eddison claims to having been taking tea (she was surreptitiously consuming liquor). Robina Redmond claims to have been using the toilet (she was in the bathroom loading a tiny pistol). Roger Curbishley claims to have been walking alone (he was actually having a tryst with his lover Davenport, one of the servants). Colonel Hugh claims he was reading military memoirs in the study (he was actually viewing erotica and fantasising about can-can dancers - which causes him to slip into a second flashback). The Doctor manages to snap him out of it, much to Hugh's embarrassment.

Agatha points out that they have nothing to go on, mentioning they need to use "the little grey cells". This prompts the Doctor to talk about his fondness for the character of Poirot, which then causes him to remembering being in Belgium once to rescue Charlemagne from an insane computer. Agatha snaps him out of this flashback and he apologises. She then points out Charlemagne lived centuries ago. The Doctor tells her that he has a very good memory, before being told he missed an important clue. The Doctor then sarcastically asks if it's the bit of paper she nicked from the fireplace, explaining that he saw her. Agatha is surprised by the Doctor having noticed this, calling him a "crafty man", much to his amusement. Agatha produces the paper she removed earlier with the letters "a-i-d-e-n", preceded by one illegible letter. It obviously spells the word "maiden", although neither she nor the Doctor is able to divine its significance. They hope Donna will be able to bring them more clues.

Meanwhile, Donna has come upon a locked door during her part of the investigation. She encounters Greeves, who informs her that Lady Eddison has kept the room shut for the last 40 years, after spending six months in it recovering from malaria following her return from India. With Donna pulling rank on him as an investigator from Scotland Yard, Greeves has no choice but to open the door for her. She then dismisses him. Inside the room, it is bare except for a few children's toys, making Donna wonder even more why it has been sealed. She then hears a buzzing from the window, commenting that the 1920s, unlike her own time, still had bees...

When she pulls back the curtain, she instead finds a giant wasp outside the window and begins yelling for the Doctor. The wasp breaks in and tries attacking her, as Donna backs up to the window. Using the magnifying glass, Donna burns the wasp with the sun's rays, allowing her to run outside just as the wasp impales its stinger in the door. The Doctor and Agatha arrive, asking what she was yelling for. Donna tells the Doctor that she encountered a giant wasp, piquing the Doctor's curiosity. Agatha dismisses the idea, thinking she was scared away by a normal, tiny insect. Donna defends herself with her usual gusto: "When I say 'giant', I don't mean 'big'; I mean flippin' enormous!", pointing to the giant stinger still embedded in the door.

The Doctor opens the door to find that the wasp has "buzzed off". Agatha tries to touch the stinger, but the Doctor tells her not to do so. He takes out a vial and pencil to collect a sample. He tells them that there are plenty of alien insects, but none should be in this galactic vector. Agatha understands some of the Doctor's words, but now thinks he's insane. Donna asks the Doctor if the wasp was harmless now that its stinger is gone; it's not. The Doctor tells her that because of its size, the wasp will be able to grow a new one. Agatha then tells him that there is no such thing as giant wasps. The Doctor tells her that she is right, but points out that the question is why it's here.

In the kitchen, Davenport is speculating about the murder with another of the servants while cooking dinner, wondering who would want to kill Professor Peach. The other servant speculates that it's what happens when a party is thrown by the rich and famous. Miss Chandrakala, who is there, dismisses the idea, telling them to get back to work. However, she then has an epiphany, realising exactly what Professor Peach had discovered in the library when he was murdered. Miss Chandrakala tells them that she must speak to Lady Eddison and rushes outside to find her. However, a figure watches from above and knocks over a stone gargoyle from the ledge. It lands on Miss Chandrakala with a loud thud. Hearing the thud and Miss Chandrakala's cry, the Doctor, Agatha, and Donna rush outside to find her slipping away. As she dies, Miss Chandrkala leaves them with a cryptic message: "The poor little child..." Seeing the wasp hovering above the building, the Doctor, Donna and Agatha give chase, Donna commenting on how the roles are reversed this time (they're chasing the monster now). Agatha is still in denial about the wasp being real, thinking it's some kind of illusion done with mirrors. They find it coming in through a skylight. The Doctor tries reasoning with it, before they barely dodge an attack. Donna gets the wasp's attention and holds up the magnifying glass to threaten it with another shot of focused sunlight. It flies into the next hall as the Doctor yells for them to hurry, and not let it return to human form. Entering the hallway, the Doctor shouts, "There's nowhere to run. Show yourself!" All of the doors open and every suspect appears, looking confused, leading the Doctor to protest "That's just cheating!".

Everyone gathers in the sitting room, where they pressure Agatha to solve the murders. However, she tells them that she is only a writer and that the Doctor is their best chance at solving the case now. She retreats to the garden. Donna follows her to try to restore her confidence. Agatha sadly admits having found her husband with another (younger) woman. Donna compares that with her own trouble with men. She also suggests that someday Agatha's books may be turned into talking films, before realising her gaffe. Agatha, however, thinks that her books may fade out of interest over time, and she will be forgotten. She then notices a box nearby that has crushed some flowers. Donna points out that nobody else would have noticed that.

They take the box to the Doctor, who is in the sitting room. The contents are full of a thief's tools, and they suspect that the Unicorn is one of the guests. Greeves arrives and gives them their drinks. Donna asks the Doctor what he found out about the venom from the stinger. Taking out the vial, the Doctor explains that the venom comes from a Vespiform, a race of aliens who have hives in the Silfrax Galaxy. However, the question remains as to why it's on Earth and acting like a character out of one of Agatha's books. Donna then asks Agatha what Miss Marple would do, pointing out the character's M.O. before again realising that she has given yet another idea to Agatha. Donna decides to have Agatha copyright Miss Marple to both of them. The Doctor then calls to Donna with a blank face, saying something is inhibiting his enzymes. He suddenly begins having convulsions; someone has poisoned his drink. Agatha deduces the poison is cyanide by the telltale smell of bitter almonds.

The Doctor rushes to the kitchen, frantically asking for ginger beer. Upon finding a bottle, he drinks some and then pours the rest on himself. Agatha tells him that as an expert in poisons, she knows cyanide is fatal. The Doctor points out that he (being a Time Lord and non-human) can stimulate the inhibited enzymes into reversal, thus curing himself. He next asks for protein, and is given walnuts. Then he mimes (very badly) a need for salt; when Donna tries to give him a bag of salt, he says pure salt is "too salty", so Agatha gives him a bottle of anchovies. Finally, he says he needs a big shock. Promptly, Donna kisses him. The Doctor exhales the poison in a cloud of smoke, saying he should detoxify more often. Agatha is flabbergasted, exclaiming, "Doctor, you are impossible! Who are you?"

Thunder and lightning have arrived by nightfall. As they are all seated in the dining room, the Doctor points out that they are still having dinner even thought two people have died. Lady Eddison asks what he would want them to do; being British, they carry on. The Doctor then tells the guests that one of them has failed to poison him. Any one of them could have put cyanide in his drink. He then mentions that it gave him an idea. When Golightly asks what it is, the Doctor responds, "Well, poison." He tells everyone that the soup has been laced with pepper. Colonel Hugh finds the extra spice delightful, but the Doctor explains that the active ingredient in pepper is piperine, traditionally used as an insecticide. At that moment, there is a flash of lightning and a thunderclap. A sudden gust of wind blows open the windows, and the candles go out. The Doctor asks the startled group to listen, and sure enough, the wasp's buzzing becomes audible again. Lady Eddison exclaims, "No...it can't be!" Agatha, meanwhile stands up and calls, "Show yourself, demon!" It does, above a painting, and the guests flee the dining room.

The Doctor, Greeves, Donna and Agatha end up in the same room, just outside the dining room. The Doctor takes a sword from a nearby coat of arms and draws it. Donna half-jokes with Greeves beside her, "Well, we know the butler didn't do it." The Doctor asks in response, "Then who did?" As he rushes back in with the sword in hand and the others in tow, the lights come back on. The wasp is nowhere to be found. Lady Eddison, who was in her seat the whole time, then notices that her necklace, "the Firestone", is stolen. Davenport then sadly says Roger's name. Robina looks over to Roger and screams in horror, but Lady Eddison's wails quickly drown her out. The Vespiform has stabbed Roger in the back, and he has slumped forward into the soup. Lady Eddison goes over to her son, crying over his death.

Later, in the sitting room, the lightning and thunder continue outside. Agatha is trying her best to cope with what's happened, with the Doctor at her side. Donna enters, feeling sorry for Davenport; he can't mourn Roger, because of the social mores of the day. Agatha then asks Donna if she inquired about the Firestone. According to Donna, it's a priceless jewel that Lady Eddison brought back from India 40 years ago. The Doctor then begins wondering why the Vespiform hasn't used its abilities to kill them all already.

Agatha tells the Doctor to stop as she knows the murderer is as human as them. The Doctor then realises that Agatha is right. He tells her that he's been so caught up in figuring out this giant wasp, that he's forgotten that she's the expert. Agatha again refuses to believe her work is any good, dismissing herself as "just a purveyor of nonsense". The Doctor tells her that the reason her books are so good is because she knows the human mind well. If anyone can solve the case, it is Agatha.

With her confidence restored, Agatha, the Doctor and Donna call the remaining four suspects together in the sitting room. The Doctor introduces Agatha, inviting her to begin. He takes a seat beside Donna. Between the two of them — the Doctor with his head rested on his hand and Donna munching a tray of grapes like one might eat popcorn — they both look ready for something big to unfold.

Agatha starts with Robina and quickly exposes her as an impostor due to her terminology, saying "toilet" instead of the upper-class "loo". They found the thief tool box below her bathroom window. Agatha concludes Robina must have thrown them out when she heard Donna was searching the rooms. "Robina" is the Unicorn! Losing her fake posh accent for an East-End one, the Unicorn reveals she stole and still has the Firestone, handing it over to the Doctor. She tells them that although she may be a thief, she's no killer. Agatha then turns her attention to the Colonel. He confesses that he does not actually need his wheelchair. He faked a disability in order to keep Lady Eddison at his side, fearing she would fall in love with another man. When asked how Agatha figured out the truth, she tells him that she didn't. Much to his embarrassment, she was simply going to say he was innocent.

Agatha picks up the Firestone and says that it has quite the history — and is far more than the Unicorn's prize. She turns to Lady Eddison, who quickly pleads her innocence. Agatha asks Lady Eddison to tell them the story of bringing the jewel back from India, then suffering from malaria and keeping herself confined in her room for six months. Then, ignoring her pleas to stop, Agatha concludes that Lady Eddison actually came back from India pregnant. She concealed it with the aid of Miss Chandrakala, her Indian maid and confidante who would eventually become her housekeeper. Colonel Curbishley asks his wife if it's true; she confesses it is, explaining she had no choice but to hide her pregnancy because of the scandal and shame it would bring to the family name. After all, "I'm British...I carry on."

The Doctor then takes control from Agatha momentarily, as they are now in his territory. He states that it was no ordinary pregnancy. Back in the dining room when she heard the buzzing, Lady Eddison had said, "It can't be" - and he wonders why. Lady Eddison tells the Doctor that he would never believe her, but Agatha tells Lady Eddison that he has opened her mind to believe many things.

Lady Eddison explains. In 1885, she was alone one night in Delhi, when she saw a purple shooting star land nearby. The next day, she met a young man named Christopher, with whom she quickly fell in love and had an affair. Christopher revealed that he was a Vespiform who took human form to study Earth, but Lady Eddison loved him so much, she didn't care. Soon after, during the great monsoon when the Jamuna River broke its banks, he drowned. However, he left her both the Firestone and pregnant. Out of shame of the scandal that her out-of-wedlock baby would bring, Lady Eddison sadly gave up the child for adoption. Donna then realises that "maiden" on the paper meant "maiden name", which Agatha verifies as the reason Professor Peach was killed. He had found out who the child was and who his parents were, by discovering the birth certificate. Agatha then tells Lady Eddison that Miss Chandrakala had (correctly) feared that Professor Peach had found out the truth. She was coming to warn her. Just as Lady Eddison is about to break down, Agatha says that she is innocent of murder. She turns it back over to the Doctor.

He pops up as Agatha sits back down and starts, "Thank you! At this point, when we consider the lies and the secrets that are key to these events..." Firstly, the Doctor points at a confused Donna. However, he simply says that she was right — the vital clue was that everything in this story has been acted out like a murder mystery. He next points at Agatha. Donna wonders if Agatha was involved, as Agatha is taken aback. The Doctor dismisses the idea quickly, saying that she simply wrote those brilliant, clever books. The Doctor then points at Lady Eddison, who is such an admirer of Agatha's works, and asks what she was doing the previous Thursday night. It turns out that Lady Eddison was reading her favourite Agatha Christie book (The Murder of Roger Ackroyd). She asks how it is relevant. The Doctor then points out that the church robbery which Golightly foiled also happened on Thursday night.

He goes on to point out how unlikely it would have been for Golightly to defeat two strong, younger men. It has also been forty years since Lady Eddison gave birth... and Golightly is 40 years old. Lady Eddison is stunned as the Doctor proclaims, "Your child has come home." He recalls Golightly's earlier statement that he was taught by the Christian Fathers, meaning that he was raised in an orphanage. The Doctor postulates that the night of the robbery, Golightly became deeply angry for the first time in his life. The genetic lock keeping him in human form was broken, and his alien biology was awakened. He transformed, frightening the thieves into submission.

The Doctor then takes the Firestone and holds it up, revealing that it is actually a Vespiform telepathic recorder. It is part of Golightly's brain and his very essence. When Golightly transformed for the first time, the Firestone also activated and beamed his full identity directly into his mind. Because Lady Eddison was both wearing the Firestone and reading a book by Agatha at the time, his template for how the world should work was distorted - to him, it was an Agatha Christie murder mystery.

After all the false starts, Donna wonders if Golightly is definitely the murderer; the Doctor confirms it. Slightly miffed, Golightly says the evening has been entertaining and asks Lady Eddison if she believes what she's heard - but buzzes on saying her name. He does it again when the Doctor asks him to repeat what he just said, before warning the Doctor not to make him angry. Golightly sneers at the others, saying that humans worship tribal "sky gods" while he is so much more. After the upload of information from the Firestone, he wanted to take what was his, inheriting the Eddison title.

Blinded by rage, Golightly focuses on Agatha Christie, asking why he shouldn't just kill everyone, as a pink light surrounds him. Completely losing his temper, Golightly transforms into his wasp form. A frantic Lady Eddison reaches out as if to hug him, but is held back by the others hold her back. At this Agatha snatches the Firestone, screaming, "No! No more murder! If my imagination made you kill, then my imagination will find a way to stop you, foul creature!"

As she runs out of the room with Lady Eddison still screaming hysterically, Golightly pursues her and the Firestone, with the Doctor and Donna following them. Agatha takes a car and drives away, yelling for Golightly to chase her, which he does. The Doctor and Donna follow the two of them in the late Professor Peach's car. The Doctor ominously warns that "time is in flux" - history may change, and it may be that tonight could be the night that Agatha Christie loses her life!

Agatha leads the creature to the Silent Pool lake. Stopping, she gets out and calls Golightly to her. As they arrive and hurry to her side, Donna realises that Agatha is controlling Golightly. The Doctor notes that Agatha is linked to his mind because his mind is based on her thought processes; Agatha replies that if she dies, then it might die with her. The Doctor tries to persuade Golightly that he was not meant to be a killer, and has the wrong template in his mind.

Donna seizes the opportunity to snatch the Firestone from Agatha's grasp and hurl it into the water. Golightly shoots over their heads, splashes into the lake and is drowned, as his father had been in the Indian monsoons forty years before. A purplish light emanates from the spot where the Firestone and Golightly sank.

Agatha gives a poetic speech as the purple light fades in the water — "Death comes as the end and justice is served." The Doctor decides to call this adventure "Murder at the Vicar's Rage"; at a look from Donna, he admits that the title needs some work. Agatha tells the Doctor there is just one more mystery left: who is he? However, she then cries out in pain and collapses. As the Doctor catches her, he realises that the two are still linked. If the Vespiform dies, so does Agatha...

However, Golightly cuts the link right before he dies. Agatha is bathed in the purple light for a few moments before she merely exhales deeply and faints. The Doctor remarks that it let her go... At the end, it chose to save her life. Donna then wonders what will happen now as the Doctor finally figures out how Agatha lost her memory. It was caused by the psychic trauma of the link between her and Golightly being severed so violently. Donna is sad that this means she'll forget meeting them. The Doctor says that they've solved their mystery and can now let — or help — history take its course. Keeping with the established timeline, the Doctor leaves Agatha's car by the lakeside, takes Agatha in the TARDIS and drops her off at the Harrogate Hotel ten days later.

As they watch a confused Agatha wander over to the hotel, Donna wonders about Lady Eddison, the Colonel and the servants, asking if they would tell anyone about what happened. The Doctor reminds her that they are far too "British" to tell such a shameful story, and that the Unicorn would have escaped back to London. Donna then wonders what will become of Agatha; the Doctor explains that she will get married again, see the world and keep writing. Donna sadly tells the Doctor that Agatha never thought her work was any good as they board the TARDIS.

In the TARDIS, the Doctor tells Donna that he thinks Agatha never quite forgot what happened as he pulls open a hatch below the TARDIS floor. He pulls out a chest, where he stores souvenirs under the letter "C". Amongst the various knick-knacks he tosses out of it (including the globe imprisoning the Carrionites and the chest emblem from a Cyberman), the Doctor produces Death in the Clouds, an Agatha Christie novel which features a gigantic wasp on the cover. Donna is shocked. The Doctor points out that as she had such a great mind that some of the details bled through — stuff her imagination could use, such as the character of Miss Marple and the basic plot for Murder on the Orient Express. However, the Doctor then has Donna look at the copyright page in front, which shows that the book is a reprint from the year five billion. Agatha Christie is quite literally the most popular writer of all time.

Donna then reminds the Doctor that Agatha never thought her work was good, but the Doctor responds, "Well, no one knows how they're going to be remembered. All we can do is hope for the best. Maybe that's what kept her writing. Same thing keeps me travelling. Onwards?" He asks Donna with a smile. "Onwards," she says, returning the smile. He pulls a lever on the console and they set off.

Cast

 * The Doctor - David Tennant
 * Donna Noble - Catherine Tate
 * Agatha Christie - Fenella Woolgar
 * Lady Eddison - Felicity Kendal
 * Revd Arnold Golightly - Tom Goodman-Hill
 * Col Hugh Curbishley - Christopher Benjamin
 * Robina Redmond/The Unicorn - Felicity Jones
 * The Hon Roger Curbishley - Adam Rayner
 * Greeves - David Quilter
 * Davenport - Daniel King
 * Professor Peach - Ian Barritt
 * Miss Chandrakala - Leena Dhingra
 * Mrs Hart - Charlotte Eaton

Locations

 * The Doctor and Donna drop Agatha off at the Harrogate Hotel.

Species

 * The Doctor brings out the Carrionite sphere (the witches' screams of captivity can be heard emanating from it), and the metal plate of a Cyberman's armour with the Cybus Industries logo stamped on it from a chest of things beginning with C.

Individuals

 * The Unicorn has stolen Lady Babbington's pearls.
 * Agatha compares the Doctor to Edward Lear.
 * The Doctor reminisces on how he had to save Charlemagne from an insane computer. Agatha reminds him that Charlemagne died centuries ago.

Technology

 * Agatha is surprised at the existence of talking pictures when Donna suggests that her books will be made into such films.

Foods and beverages

 * Donna orders a sidecar and the Doctor orders a lime soda at the garden party.
 * Donna and Agatha Christie drink orange juice and Golightly tries to poison the Doctor by slipping cyanide into his lemonade.
 * The Doctor consumes ginger beer, walnuts and anchovies to cure himself of the cyanide poisoning.
 * Donna mentions or proposes milk and milkshake among the wrong ingredients to cure the Doctor's poisoning.
 * The Doctor laces the soup with pepper in an attempt to draw out the wasp creature because the piperine in pepper is a traditional insecticide.
 * While trying to work out what the Doctor is miming for when he's been poisoned, Donna wonders if he wants a Harvey Wallbanger cocktail because she thinks he's attempting to mime a shaken drink (he's miming a salt-shaker).

Cultural references

 * When trying to understand what the Doctor needs to overcome his poisoning, Donna mentions the songs Mammy and Camptown Races.

Story notes

 * In terms of principal photography, this is the second story shot in the fourth series.
 * A different opening and conclusion to the episode were filmed. The two scenes take place in the mid-1970s and feature another actress playing the aged Christie, who begins the episode by having flashbacks and dreams relating to her adventure of some 50 years earlier. The ending of the episode featured the Doctor and Donna visiting Christie, upon which she begins to remember what happened and is shown a copy of the facsimile edition of Death in the Clouds from the year 5 billion. When it was decided to abandon the framing sequence, this second scene was remounted in the TARDIS. Both scenes were presented to the public for the first time in the 2008 DVD box set of Series 4.
 * Some of these characters resemble Cluedo characters.
 * Clemency Eddison resembles Mrs Peacock
 * Robina Redmond resembles Miss Scarlet
 * Colonel Hugh Curshiby resembles Colonel Mustard
 * Professor Gerald Peach resembles Professor Plum
 * Miss Chandrakala resembles Miss White
 * Reverend Arnold Golightly resembles Reverend Green
 * In addition, some variations on the game include a "Miss Peach" character, including the colour of the unfortunate professor from this episode.
 * Other references to Cluedo include the lead pipe and revolver as murder weapons, as well as the first murder commited in the library.
 * David Tennant's father Sandy MacDonald makes a cameo as a footman. The Doctor Who Confidential Nemesis showed that he was visiting David on set and was offered a part on the spot — and he confessed he was glad he didn't have to learn lines.
 * Graeme Harper's penchant for including a distorted image of a main character is present in this story. Though not included in every single story he's directed for BBC Wales, it's seen often enough to be considered something of a directorial "signature". Similar distortion is achieved through the use of other magnifying glasses in Army of Ghosts and Utopia, a curved window in Journey's End, and with mirrors in Turn Left. This "signature" has appeared as early as The Caves of Androzani, in which the shot of the Fifth Doctor's face is electronically distorted as he starts to regenerate. This time, it's Donna's face that gets "the Harper treatment".
 * The Agatha Christie book the Doctor produces from the trunk underneath the Tardis is a Fontana Paperback fascimile edition, published in 1957. Presumably the Year 5,000,000,000 copyright page was somehow transplanted in.
 * The Doctor's description of his cyanide poisoning as involving "inhibited enzymes" approximates the actual mechanism of cyanide toxicity. Cyanide inhibits the enzyme aa3 by binding to the iron that it contains, reducing the ability of the cell use oxygen to create the compound ATP, used for cellular energy transfer. Since 95% percent of ATP production in the human body (and presumably also Time Lords) is aerobic, mortality via energy starvation quickly follows. The Doctor's method of uninhibiting the enzymes, however, is not based on a real remedy (for humans, in any case).
 * The disappearance of bees is frequently mentioned during Series 4. Its significance is eventually revealed in The Stolen Earth. In the real world, both bumble bees and honey bees are disappearing from the globe, a.
 * In The Writer's Tale, Russell T Davies talked about how the ending was a rewritten one — originally, the Doctor was to have rammed the Vespiform into the lake with the car that he and Donna had commandeered, but Tennant objected because he was concerned that that ending would portray the Doctor as a murderer.
 * In The Writer's Tale, Davies also reveals that during his rewriting of the script he entertained himself by inserting as many references to Christie novels as possible. He originally inserted a reference to Ten Little Niggers (also known as Ten Little Indians and And Then There Were None) before deciding it was "too risky" to keep in:
 * Donna: It's like "Ten Little" -
 * The Doctor: Niggles aside, we'd better look in the library.
 * Doctor Who Confidential Nemesis shows that at least one of the classic cars that was used during filming was a Morris Cowley, which Christie was actually driving in 1926.
 * There have been multiple implicit references throughout the history of the programme to interspecies intercourse/offspring. The Doctor previously implied to Rose that the willingness to mate with other species would become a significant factor in humans expanding out into the universe. (TV: The Doctor Dances) A few such examples include: King Peladon, who was half Pel and half human; human/Catkind offspring encountered on New Earth; and Cassandra referring to herself as the only "true human" left by her time period.
 * With this episode, Donna joined the list of companions to kiss the Doctor (or be kissed by him) on the lips. By this time, on-screen kisses included: Dr Grace Holloway, shortly after his seventh regeneration; Jack Harkness, who kissed the Doctor (and Rose) farewell whilst on the Gamestation; the Doctor kissing Rose Tyler later in the same episode; Cassandra kissing the Doctor while possessing Rose Tyler's body; Jackie Tyler kissing the Doctor three times when he and Rose returned home in the middle of the ghost crisis; the Doctor kissing Martha Jones to confuse the Judoon's scanners; and Astrid Peth kissing the Doctor twice whilst on the Titanic. Unlike the other kisses, however, Donna's kiss is not a romantic gesture, but intended to induce a shock in him as part of his antidote to cyanide poisoning (she has always insisted she is not interested in him, unlike Rose and Martha).
 * The episode is littered with references to Agatha Christie novels and short stories, including:
 * Donna mentions Miss Marple and the story of the murder on the Orient Express before Agatha Christie has written about them, and may therefore have inadvertently caused the creation of them.
 * When the Doctor is poisoned Agatha is heard to utter the words "sparkling cyanide", which was the title of one of her future novels.
 * The Colonel faking his inability to walk and the Unicorn posing as Robina Redmond are ideas that appeared in the Christie novel After the Funeral.
 * The Man in the Brown Suit: The Doctor wears a brown suit throughout the course of the episode; in the original teaser sequence the elderly Agatha wonders who "the man in the brown suit" was that had haunted her for fifty years.
 * Dead Man's Folly: Miss Chandrakala's description of the Professor's book (as in "a dead man's folly"; not the actual name of the book)
 * The Body in the Library: Donna's remark about Professor Peach's death
 * Cat Among the Pigeons: A servant's remark about the murder
 * Nemesis: One of Agatha's nicknames for the murderer; also used for the episode title of the Doctor Who Confidential episode looking at The Unicorn and the Wasp
 * The Secret Adversary: Another of Agatha's nicknames for the murderer
 * N or M?: The Doctor's question to Agatha about the piece of paper she found
 * Murder on the Orient Express: Donna mentions it to Agatha Christie, unaware that the book hasn't been written yet
 * Why Didn't They Ask Evans?: The Professor's remark before he dies is "Why didn't they ask... heavens!" (the book's title refers to the murder victim's last words)
 * They Do It With Mirrors: Agatha's explanation about the giant wasp, believing it to be a hoax
 * Appointment with Death: Lady Eddison says Chandrakala had "an appointment with death" when mourning over her son's death
 * Cards on the Table: The phrase the Colonel used when he asked Agatha about what she knew about the mystery
 * Sparkling Cyanide: Agatha Christie says after detecting cyanide in the Doctor's lemonade
 * Yellow Iris (a short story included in The Regatta Mystery and Other Stories): at the dinner where the Doctor spikes the soup with pepper to smoke out the Vespiform, there is a vase of yellow iris as a table centrepiece. It was later expanded to novel length as Sparkling Cyanide.
 * Crooked House: How Agatha described the history of the four remaining suspects - and consequently, Eddison Manor
 * Endless Night: How the Doctor described the evening of the events at the start of the debriefing
 * Taken at the Flood: Lady Eddison said Christopher "was taken with the flood"
 * The Moving Finger: The Doctor pointed at Donna, Agatha and then Lady Eddison at the start of his portion of the debriefing and literally said, "The moving finger points at you..." when he turned to Lady Eddison, even going so far as moving his finger in a circular motion
 * The Murder of Roger Ackroyd: Lady Eddison is reading the book when the Firestone activated. Moreover, Lady Eddison's son is named Roger.
 * Death Comes as the End: Agatha said "Death comes as the end and justice is served" after Golightly's death
 * The Murder at the Vicarage: The Doctor called the events "Murder at the Vicar's Rage" after Golightly's death, though after Donna recognised his verbal mangling, he admitted it could do with some work
 * Death in the Clouds: In the TARDIS, the Doctor showed Donna a facsimile copy of this book published in the year 5 billion, with the Fontana edition cover showing a giant wasp attacking a plane. In the original final scene when the Doctor shows the elderly Agatha Christie the facsimile edition, she says that the cover with the giant wasp gave her a shock when she first saw it. In the book, the murder victim is originally thought to have died from a wasp sting.
 * Ten Little Niggers (also known as Ten Little Indians or And Then There Were None): The main enemy is a bee-like creature, and the Doctor is poisoned with cyanide. One of the murders in the novel involves a bee and a cyanide injection.

Ratings

 * 8.4 million (official)

Myths and rumours

 * Due to actress Billie Piper being seen on set it was thought that the "Unicorn" was in fact Rose Tyler. This was false.
 * Brief glimpses of the giant wasp in this episode were included in the first Series 4 trailer, leading to speculation (since the view is too short to definitively identify the species) that this was a reference to the ongoing theme of bees. This was also false.
 * Similarly, the brief glimpse in the trailer of the actress who plays Clemency Eddison, who resembles the actress who played Lady Cassandra in Series 1 and 2, led to speculation that Cassandra would appear in this episode. This was also false.
 * At the beginning of the episode, the phonograph plays a march and the first notes are reminiscent of the Master's theme. This was true.

Studio

 * Upper Boat Studios, Trefforest

Location

 * Llansannor Court, Vale Of Glamorgan
 * Tredegar House, Newport
 * Pen Y Lan Road, Newport
 * Cefn Mably Lakes, Cefn Mably
 * Hensol Castle, Hensol

Production errors

 * At two points during the final scenes, actors mistakenly lapse into their natural accents. When Agatha says she was about to say Col. Curbishley was innocent, Fenella Woolgar lapses into her regular Estuary English accent, while when the Tenth Doctor tells Golightly at the lakeside, "Don't hurt her!", David Tennant lapses into his natural Scottish accent.
 * At one point in the confrontation scene, Agatha was looking at the Doctor without holding her glass of drink. The scene then cuts to a wider shot showing her putting her drink down on the table before looking up again.

Continuity

 * The Doctor suggested to Martha Jones in TV: Last of the Time Lords, that they take a trip to see Agatha Christie, but was unable to due to Martha leaving at the time. Ironically, this is not long after Martha has left again (at the end of The Doctor's Daughter).
 * This is the third time a famous author or playwright has appeared in the revived series: Charles Dickens, (TV: The Unquiet Dead) William Shakespeare (TV: The Shakespeare Code) and now Agatha Christie.
 * The Doctor displays the ability to tell a time period by smell (which may have been faked due to the vintage car in the driveway). However, he previously displayed this ability in WC: Scream of the Shalka. Such an ability is also referenced in AUDIO: The Eye of the Scorpion (although, in this, the Doctor admits to having seen ancient texts on the wall behind the TARDIS). Later, in TV: The Eleventh Hour, he demonstrates a similar ability to determine age by taste (on the shed in Amy Pond's garden).
 * Donna states that solving a mystery with Agatha Christie is like "...meeting Charles Dickens, and he’s surrounded by ghosts. At Christmas", to which the Doctor replies, "Well...", recalling his adventure involving those exact circumstances. (TV: The Unquiet Dead)
 * The Doctor and Donna are once again mistaken as a couple. This has previously occurred in TV: The Fires of Pompeii, Planet of the Ood and The Doctor's Daughter. This time, however, Agatha has already worked it out (no wedding rings).
 * Donna compares her late fiancé Lance Bennett's conspiracy with the Empress of the Racnoss to Agatha's marital troubles. (TV: The Runaway Bride)
 * Agatha Christie is one of several people to demand to know who the Doctor is, only to receive no real reply. Charles Dickens, Solomon, Trish Webber, the passengers of the Crusader 50, and Metella are but a few of the others who have fruitlessly questioned the Doctor's identity. (TV: The Unquiet Dead, Fear Her, Daleks in Manhattan, The Fires of Pompeii, Midnight)
 * This is the third time that the Doctor has told a companion not to imitate a period dialect. The first was Rose doing a (poor) Scottish accent in TV: Tooth and Claw and the second was Martha imitating Elizabethan English in TV: The Shakespeare Code. This time Donna attempts an over-exaggerated upper class accent. Though not exactly a period dialect, the Doctor also tells Martha to stop when she tries to imitate a pirate voice in TV: The Infinite Quest.
 * In AUDIO: Terror Firma the Eighth Doctor claimed that Agatha Christie travelled with him.
 * Donna gives Agatha ideas for her future work in a similar manner to how the Doctor did with William Shakespeare. (TV: The Shakespeare Code) Unlike Shakespeare, Agatha has no conscious memory of these conversations.
 * Lady Eddison calls Chandrakala her "faithful companion." The Master previously used the same two words for his wife Lucy Saxon (although almost certainly ironically). (TV: The Sound of Drums)
 * Though she is probably simply using the stereotypical name for an alien planet, Donna makes reference to the planet Zog, which is later seen in TV:The End of Time. There is also the chance that Zog was their original destination and the TARDIS took them to the wrong place, which is evident from the Doctor having to deduce what time they were in at the beginning of the episode, which would be unnecessary if he chose to visit Earth in said timeline.
 * What happens to Agatha Christie is not dissimilar to Donna's own ultimate fate; unlike Agatha, the consequences of Donna regaining her memories are far worse. (TV: Journey's End)
 * When Donna was investigating the locked room, she heard the buzzing of the wasp before she saw him. She commented aloud that in 1926, there are still bumble bees. The disappearance of bees is frequently mentioned during Series 4. Its significance is eventually revealed in TV: The Stolen Earth.
 * When Donna later begins to remember her adventures with the Doctor, she asks Wilfred why she can see a giant wasp, a reference to the Vespiform. (TV: The End of Time)
 * Sir Hugh mentions the Relief of Mafeking during the Boer Wars, at which the First Doctor was present. (TV: The Daleks' Master Plan, TV: The Invasion of Time)
 * The Doctor has a flashback to when he saved Charlemagne from "an insane computer" in Belgium. This adventure is later explored in PROSE: The Lonely Computer.
 * Donna is terrible at understanding the Doctor's miming. Another companion has similar trouble in GAME: TARDIS, when Amy takes a while to work out what the Eleventh Doctor is trying to tell her.

Home video releases

 * This story was released in the Series 4 DVD box set in November 2008 along with the rest of the series.
 * It was released as Series 4 Volume 2 in a vanilla edition with The Sontaran Stratagem, The Poison Sky and The Doctor's Daughter.