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 Agatha Christie in 1926, giving a possible, yet unlikely answer as to why she vanished.

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

Plot
The TARDIS materialises outside a country estate, hidden by trees; the Doctor and Donna Noble depart. 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 and is 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, and Peach decides to go to the library to do some research on his own. Golightly tells Professor Peach that constantly working will be the death of him; they're at a party, he should try to relax.

Watching from the side of the house, Donna tells the Doctor to forget about Planet Zog as a party in the 1920s is much more fun. The Doctor points out that they're not invited, but then pulls out his psychic paper, saying "oh, I forgot. Yes we are". They head back to the TARDIS so Donna can find a dress from the 1920s to wear.

In the library, Professor Peach discovers a secret amongst the papers. 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, bludgeons the professor to death.

Back outside, the Doctor is waiting outside the TARDIS for Donna to finish changing, reminding her that they'll be late for cocktails. Donna comes out in a dress and the Doctor says she looks lovely. They head for the front lawn, and after greeting by Lady 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 precautious because the Unicorn is about. The Doctor, at first, mistakes this for an actual unicorn before Lady Eddison clears up the confusion by explaining the Unicorn is a jewel thief who is on the loose and has just struck again.

They then meet Lady Eddison's wheelchair-handicapped husband, Colonel Hugh Curbishley, and their son Roger. Donna is confused why Lady Eddison has a different surname and the Doctor explains that the Eddison title descends through Lady Eddison; one day Roger will be Lord Eddison. 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. Next to arrive is Reverend Golightly, who Lady Eddison congratulates on the apprehension of two boys who tried robbing the church last Thursday. Following him is fashion model Robina Redmond. Last to come is British mystery writer, Agatha Christie, whom the Doctor is ecstatic about meeting as she is another of his favourite authors. Still a young writer, she has recently published her sixth novel; the Doctor and Donna are both impressed. 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 then shows Donna the date - 8 December 1926, the day Agatha Christie disappeared. He explains that Agatha had just discovered that her husband is having an affair. Being British, these people would normally just carry on, but this time, that didn't happen. Tomorrow morning, her car will be found abandoned by the side of a lake. In ten days, Agatha will turn up in a hotel in Harrogate with no memory of what happened.

Miss Chandrakala comes back, having found Professor Peach's body. The Doctor and Donna race to the library, 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 peice of paper from the fireplace, but the Doctor notices in her reflection in the bookcase. Everyone else comes in and decides to call the police. However, the Doctor uses his psychic paper to identify himself as a Scotland Yard inspector and has everyone leave with Agatha keeping them in the sitting room for questioning. Donna comments on the similarity to the board game Clue and says that this happening with Agatha Christie around is odd, off-handedly mentioning "Charles Dickens surrounded by ghosts at Christmas."

The Doctor hints he experienced it as he finds "morphic residue" on the floor while examining the scene - a by-product of shapeshifting; one of the guests isn't really human. The Doctor tastes the residue, trying to figure out what left it as he and Donna walk to 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 that it's one of her best books, but the Doctor tells her "not yet", hinting to keep quiet. The Doctor tells Donna that he and Agatha will question the suspect, handing her a magnifying glass to investigate the rooms upstairs. The Doctor expresses joy at being able to solve a murder mystery with Agatha. She reluctantly agrees, unhappy at the Doctor's casual attitude toward the murder.

During the interviews, while the guests recount their stories of what they were doing at 4:15 PM, flashbacks reveal that each is hiding something, except for Reverend Golightly (who claims to have been unpacking in his room): Lady Eddison claims to having been taking tea, though she was surreptitiously consuming liquor AKA "liquid courage". Robina Redmond claims to have been using the toilet, but was in the bathroom loading a tiny pistol. Roger Curbishley, Lady Eddison's son, claims to have been walking, alone, when he was attending a tryst with his lover, Davenport, one of the servants. Colonel Hugh claims he was reading military memoirs in the study, though he was viewing erotica while 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". The Doctor goes on about how he likes Poirot and begins remembering about how he had been to Belgium, where an insane computer had kidnapped Charlemagne. 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. Agatha is surprised by the Doctor noticing her, 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 comes upon a locked door during her part of the investigation and encounters Greeves, who informs her that Lady Eddison has commanded the room be shut for the last 40 years after recovering from malaria in it. On Donna mentioning being an investigator from Scotland Yard, Greeves has no choice but to open the door for her; she 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 still have 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 while Agatha dismisses the idea, thinking she was scared away by a normal, tiny insect. Donna defends herself "When I say 'big', I don't mean tiny; I mean flipping enormous!" and points to the stinger still embedded in the door.

The Doctor is amazed/shocked and tells Donna to let him have a look and opens the door to find that the wasp has "buzzed off". Agatha tries to touch the stinger, but the Doctor tells her not to as he takes out a vial and pencil to collect a sample, saying 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, wondering about who would want to kill Professor Peach. Miss Chandrakala is also there, and dismisses it until she suddenly realises exactly what Professor Peach was doing. She tells them that she must speak to Lady Eddison and rushes off outside. However, she is killed when an unseen figure hurls a gargoyle from a ledge--and hits her head.

Hearing her scream, the Doctor, Agatha, and Donna rush outside; Miss Chandrakala dies, cryptically saying "The poor little child..." Seeing the wasp hovering above the building, the Doctor, Donna and Agatha gives chase with Donna commenting on how the roles are reversed this time. Agatha is still in denial about the wasp being real, thinking it's some kind of illusion. They find it coming in through a skylight and 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. It flies into the next hall as the Doctor yells for everyone to hurry and not let it return to human form. When the Doctor shouts "show yourself," all of the doors open and every suspect appears; "that's just cheating".

Everyone gathers in the sitting room, where they pressure Agatha to solve the murder(s). 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 goes out with her and compares her similar trouble with men along with foreknowledge that someday her books may be turned into films. Agatha, however, thinks that her books may fade out of interest and she will be forgotten until she notices a box that has crushed some flowers.

They take the box to the Doctor, who is in the sitting room; the contents are full of a thief's tools, possibly the Unicorn. Greeves arrives and gives them their drinks as Donna asks the Doctor what he found out about the venom from the stinger. Taking out the vile, 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 here 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. and again noticing that she has given yet another idea to Agatha; Donna decides to have Agatha copyright Miss Marple to her. The Doctor then calls to Donna with a blank face, saying something is inhibiting his enzymes. Suddenly, the Doctor starts to have convulsions, realising someone has poisoned his drink as Agatha deduces the poison is cyanide.

The Doctor rushes to the kitchen, 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 can stimulate the inhibited enzymes into reversal, curing himself. He then asks for protein (in walnuts), salt (contained in anchovies; pure salt is apparently "too salty") followed by a big shock, which comes from a kiss from Donna. The Doctor then exhales the poison in a cloud of smoke. Agatha calls the Doctor impossible and then asks who he really is.

At dinner that night, 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; they are British, they carry on. The Doctor then tells the guests that one of them has failed to poison him and that anyone of them could have put cyanide in his drink before mentioning it gave him an idea. Golightly asks what it is and the Doctor responds "poison, drink up" as the diners are eating soup. Donna chokes on her soup for a moment before the Doctor tells everyone that the soup has been laced with pepper. Colonel Hugh finds the extra spice delightful before the Doctor continues and explains that the active ingredient in pepper is pepperine, normally used as an insecticide, asking if anyone has "the shivers".

A buzzing is heard. Lady Eddison exclaims, "It can't be!" The lights are blown out by a sudden wind and everyone spreads out, much to the Doctor's annoyance. Agatha demands that the "demon" show itself, which it does, above a painting. The Doctor and Greeves pull Donna and Agatha out of the dining room. As the Doctor takes a sword from a coat of arms, Donna jokes "at least we know the butler didn't do it". The Doctor opens the door, asking "Who did?" The lights come back on and the wasp is nowhere to be found. Lady Eddison then notices her necklace, 'The Firestone', is stolen. Davenport then sadly says Roger's name. Robina looks over to Roger and screams in horror; the Vespifrom has stabbed Roger in the back. Lady Eddison goes over to her son, crying over his death.

Later, in the sitting room, Agatha is trying her best to cope with what's happened with the Doctor at her side. Donna enters, feeling sorry for Davenport as he can't mourn Roger. Agatha then asks Donna if she inquired about the Firestone; it is a priceless jewel that Lady Eddison brought back from India 40 years ago. The Doctor then begins wondering about the Vespiform; "It can sting; it can fly. So, why's it playing this game?" 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's been so caught up in figuring out this giant wasp, that he's forgotten that she's brilliant. Agatha just says that she doesn't believe her work is any good before the Doctor points out that the reason her books are so good is because she knows the human mind well; she knows what tiny little things can lead to murder, saying that "if anyone can solve this case, it's you".

With her confidence restored, Agatha, the Doctor and Donna call the remaining four suspects together in the sitting room. The Doctor introduces Agatha and lets her begin. Starting with Robina Redman, who she quickly exposes as an imposter due to her terminology and the fact they found the thief tool box below her bathroom window. Agatha points out Robina must have thrown them out when she heard Donna was searching the rooms; "Robina" is the Unicorn. Losing her fake accent, the Unicorn reveals she still has the Firestone and hands it over to the Doctor; she is also not the murder. Agatha then turns her attention to the Colonel, who is now revealed to not need a wheelchair; he only pretended to keep Lady Eddison at his side as he feared she would fall in love with another man. When asked how she figured out the truth, Agatha tells the Colonel that she didn't and she was going to say he was innocent.

Agatha picks up the Firestone and asks Lady Eddison to explain how she brought it back from India, then suffered from malaria and kept herself confined in her room. Agatha concludes that Lady Eddison came back from India pregnant and concealed it with the aid of Miss Chandrakala. The Doctor then says "it was no ordinary pregnancy", prompting a confused Lady Eddison to wonder how he knew. Taking control from Agatha as they are in his territory, the Doctor asks Lady Eddison why she said "it can't be" back in the dining room. Though Lady Eddison tells the Doctor that he would never believe her, Agatha tells Lady Eddison that he has opened her mind to believe many things.

Lady Eddison explains: it was 1886, India; she was alone one night when she saw a purple shooting star land nearby. The next day, she met a young man named Christopher, whom she quickly fell in love with and had an affair. Christopher revealed he was a Vespiform who took human form to study Earth; Lady Eddison loved him so much, she didn't care. However, he died soon after, during a monsoon, but he left her the Firestone. Out of shame of the scandal her out-of-wedlock baby could bring, Lady Eddison sadly gave him up for adoption. Donna then realised that "maiden" on the paper meant "Maiden name", which Agatha verifies as the reason Professor Peach was killed; he found out who the child was and who his parents were. Agatha notes that Lady Eddison is innocent of murder and turns it back over to the Doctor.

The Doctor points at Donna, saying that she was right, everything is being acted out like a murder mystery, which causes him to next point at Agatha, explaining that she wrote those clever books. Donna then wonders if Agatha was involved, but the Doctor dismisses the idea, telling her that Lady Eddison is her biggest fan. The Doctor then asks what she was doing the previous Thursday and learns that Lady Eddison was reading her favourite Agatha Christie book (The Murder of "Roger" Ackroyd) before being asked how it is relevant. The Doctor then points out that the failed robbery that Golightly foiled at the church also happened on Thursday.

It would be impossible for Golightly to defeat two strong men, and it has been forty years since Lady Eddison gave birth… and Golightly is celebrating his 40th birthday. Also, Golightly said earlier that he was taught by the Christian fathers, meaning that he was raised in an orphanage. The Doctor notes that Golightly got angry (a proper deep anger for the first time in his life), and he transformed for the first time in his life, which allowed him to frighten the thieves into submission. His alien biology was awakened. Also, the Firestone is not a jewel at all, but a Vespiform telepathic recorder, part of Golightly's very essence. When Golightly activated, the Firestone beamed his identity directly into his brain. He also accidentally received the works of Agatha Christie as his template for how the world should work because Lady Eddison was thinking about the plots: hence the style of the murders.

Donna then wonders if Golightly is the murderer and the Doctor answers "yes". Slightly miffed, Golightly says the evening has been entertaining and asks Lady Eddison if she believes what she's heard, buzzing on her name. The Doctor asks what he said and Golightly buzzes "Lady Eddison" again before the Doctor tells him he's buzzing. Golightly warns the Doctor not to make him angry before the Doctor asks why. Walking away from everyone, Golightly sneers at them, saying that humans worship tribal "sky gods" while he is so much more. After the upload of information, he wanted to take what was his (inheriting the Eddison title).

Becoming blinded by rage, Golightly focuses on Agatha Christie, asking why he shouldn't just kill her as pink light surrounds him. Completely losing his temper, Golightly transforms into his wasp form, prepared to kill everyone. Agatha snatches the Firestone, screaming that her mind--that which had taught the Vespiform to kill--will find a way to destroy him. Golightly pursues her, as her thought patterns are linked to him through the Firestone. She takes a car and drives away, hysterically repeating "It's all my fault." The Doctor and Donna follow her in the late Professor Peach's car, with the Doctor ominously warning that "time is in flux" - that there is no guarantee that Agatha will survive, which could leave history severely damaged.

Agatha leads the creature to a lake, where she is "the honey in the trap", calling Golightly to her. The Doctor and Donna arrive and he notes that Agatha is linked to his mind. She plans to kill herself in order to kill Golightly. The Doctor tries to persuade Golightly that he was not meant to kill, but he does not listen. Donna seizes the opportunity to snatch the Firestone and hurl it into the water. Golightly pursues it and drowns, as his father had, in the Indian monsoons forty years before. "How do you kill a wasp? Drown him, like his father" Donna says coldly. The Doctor then points out that Golightly couldn't control himself.

Agatha then wonders who the Doctor is before collapsing in his arms in pain. The Doctor realises that the two are still linked and if it dies, so does Agatha. However, Golightly cuts the link right before passing, which causes Agatha to pass out, much to the Doctor's surprise. 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. Keeping with the established timeline, the Doctor takes Agatha in the TARDIS and deposits her at the Harrogate Hotel ten days later.

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 too British to tell such a shameful story and that the Unicorn escaped and managed to get back to London. Donna then wonders what will become of Agatha. The Doctor explains that she will get married again and continue writing her books. Donna 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 and pulls out a chest. Amongst the various knick-knacks he tosses out of it, the Doctor produces Death in the Clouds, a novel which features a gigantic wasp on the cover. Donna is shocked as the Doctor points out that she had such a great mind, some of the details bled through, stuff her imagination could use, such as Miss Marple; Donna remarks she should have had Agatha sign a contract. However, the Doctor then has Donna look at the copyright page in front, which shows that the book is 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. The Doctor tells her nobody knows how they're going to be remembered, something that keeps him travelling. He asks Donna if they should continue onwards and she agrees as he pulls a lever on the console.

Cast

 * The Doctor - David Tennant
 * Donna Noble - Catherine Tate
 * Agatha Christie - Fenella Woolgar
 * Lady Clemency Eddison - Felicity Kendal
 * Reverend Golightly - Tom Goodman-Hill
 * Colonel 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 and a Cyberman logo from a chest of things beginning with C.

Individuals

 * With this episode, Donna became the most recent companion 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, Donna's kiss is not a romantic gesture, but intended to induce a shock in him as part of his antidote to cyanide poisoning.
 * Donna compares Agatha's marital troubles to her own fiance's conspiracy with the Empress of the Racnoss. (DW: The Runaway Bride)
 * Donna sarcastically comments on the likelihood of meeting Charles Dickens and "ghosts" on Christmas day - not realising the Doctor had done just that back in Christmas week 1869. (DW: The Unquiet Dead)

Technology

 * The Doctor has a flashback to saving Charlemagne from "an insane computer" in Belgium. This adventure is later explored in the short story "The Lonely Computer".

Food and beverages

 * Donna orders a sidecar and the Doctor orders a lime soda.
 * 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.
 * The Doctor laced the soup with pepper.
 * Donna mentions the Harvey Wallbanger cocktail at one point when the Doctor is being poisoned.

Real world

 * The Firestone is a reference to the Wilkie Collins book The Moonstone (1868), one of the earliest works of detective fiction.

References to Agatha Christie novels & short stories

 * Donna mentioned Miss Marple and the story of the murder in the Orient Express before Agatha Christie has written about them, and may therefore have inadvertently caused the creation of them.
 * Another potential influence occurs when the Doctor is poisoned and Christie is heard to utter the words "sparkling cyanide", which happens to be the title of her well-known 1945 novel.
 * The Colonel faking his inability to walk and the Unicorn posing as Robina Redmond are ideas taken from the Christie novel After the Funeral.
 * Lady Eddison is depicted reading The Murder of Roger Ackroyd, an early Agatha Christie novel where the murderer is actually a doctor.
 * Novel titles referenced in the episode are:
 * Dead Man's Folly: Mrs. 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 the professor's death
 * Cat Among the Pigeons: a servant's remark about the murder.
 * Nemesis: Agatha's nickname for the murderer;
 * The Secret Adversary: Agatha's nickname 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 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 lemonade
 * Crooked House: How Agatha describes the history of Eddison Manner.
 * Endless Night: how the Doctor describes the current murder mystery
 * Taken at the Flood: Lady Eddison said "He was taken at the Flood", referring to her Vespiform lover
 * The Moving Finger: the Doctor phrases "the moving finger points" when interrogating the suspects
 * The Murder of Roger Ackroyd: the book Lady Eddison was reading when the Firestone activated
 * Death Comes as the End: Agatha says "Death comes as the end, and justice is served" after Golightly's death
 * The Murder at the Vicarage: the Doctor says "Murder at the Vicar's Rage" after Golightly's death, though he believes it could do with some work
 * Yellow Iris (a short story included in The Regatta Mystery and Other Stories): at the dinner where the Doctor spikes the soup with pepper, there is a vase of yellow iris as a table centrepiece. It was later expanded to novel length as Sparkling Cyanide.
 * In Russell T Davies' rewrite of the script, he inserted a reference to Ten Little Niggers (also known as Ten Little Indians and And Then There Were None), but deleted it

Games

 * Professor Peach is killed with a lead pipe in the study, a reference to the game Cluedo. (Known as Clue in North America)

Movies

 * After Donna has the sealed room unlocked, she tells the butler to "buttle off," possibly a reference to the Cluedo Play or Movie.

Story notes

 * This episode is the 50th episode of the revived series of Doctor Who.
 * 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.
 * David Tennant's father makes a cameo as a footman.
 * 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 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 1987. Presumably the Year 5,000,000 copyright page was somehow transplanted in.
 * Felicity Kendall, who plays Lady Eddison, was one of the stars of the classic BBC situation comedy, The Good Life. Her co star, Richard Briers appeared in DW: Paradise Towers as the Chief Caretaker, and in the Torchwood episode A Day in the Death as dying millionaire Henry Parker.
 * Christopher Benjamin had previously appeared in DW: Inferno and The Talons of Weng-Chiang.
 * 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).

Ratings
to be added

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 disappearing 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

 * According to this story, Agatha Christie went missing around summer. But in real life, she disappeared in December, making this story not completely canonical to our Universe's perspective.
 * When Donna Noble mentions "talking pictures," Agatha Christie reacts as if she has no idea what they are. This episode is set in 1926, the year before the first full length "talkie," The Jazz Singer, was released, but "talking picture" shorts had been exhibited for some time prior to that, and someone as knowlegeable as Agatha Christie would certainly have known they existed.

Continuity

 * The Doctor suggested to Martha Jones in DW: Last of the Time Lords, that they take a trip to see Agatha Christie, but was unable due to Martha leaving at the time. Ironically, this is not long after Martha has left again.
 * During the final scene, the Doctor produces a replica Agatha Christie paperback book printed in the far future. Although not an obvious cliffhanger on the order of the ending to many First, Second and Fifth Doctor stories, it does nevertheless establish the fact that traditionally bound books are still produced long after electronic editions would presumably have made them obsolete. In this sense, it may be seen as at least narratively linked to the next episode broadcast DW: Silence in the Library, which begins with the Doctor discussing how printed books continue on as a valuable resource into the far future.
 * This is the third time a famous author or playwright has appeared in the revived series: Charles Dickens (DW: The Unquiet Dead), William Shakespeare (DW: 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 BFA: The Eye of the Scorpion (although, in this, the Doctor admits to having seen ancient texts on the wall behind the TARDIS). Later, in DW: The Eleventh Hour, he demonstrates a similar ability to determine age by taste.
 * Donna stated 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/ (DW: The Unquiet Dead)
 * The Doctor and Donna are once again mistaken for to be a couple. This has previously occurred in DW: The Fires of Pompeii, Planet of the Ood and The Doctor's Daughter.
 * When Doctor remembers being in Belgium, he is carrying a bow and quiver, as he was when he met Sally Sparrow. (DW: Blink) It is possible that the adventure to save Charlemagne from "an insane computer" happened at around the same time. (WC: The Lonely Computer) It is also possible that the bow and arrows were re-used in either incident (first obtained in one incident and re-used in the other) because when Sally Sparrow encounters the Doctor and Martha with the bow and arrows, Martha mentions 'hatching time' and 'migration' inferring that they need the bow and arrows to shoot (possibly) some sort of space-bird, so the two incidents couldn't have happened right next to each other.
 * Donna compares her fiancé Lance Bennett's conspiracy with the Empress of the Racnoss to Agatha's marital troubles. (DW: The Runaway Bride)
 * The Doctor brings out the Carrionite sphere and a Cyberman logo from a chest of things beginning with C. (DW: Rise of the Cybermen / The Age of Steel, The Shakespeare Code)
 * This is not the first occurrence of interspecies intercourse. There are many references throughout the series, including a reference in DW: The Doctor Dances to it being one way in which humans so quickly propagated themselves throughout the galaxy "So many species, so little time")
 * 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 Caecilius Iucundus are but a few of the others who have fruitlessly questioned the Doctor's identity.
 * 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 DW: Tooth and Claw and the second was Martha imitating Elizabethan English in DW: 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 DW: The Infinite Quest
 * In BFA: Terror Firma the Eighth Doctor claimed that Agatha Christie travelled with him.
 * Donna gives Agatha ideas in a similar manner to how the Doctor did with William Shakespeare. (DW: The Shakespeare Code)
 * Lady Eddison calls Chandrakala her "faithful companion." The Master previously used the same two words for his wife Lucy Saxon. (DW: 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 DW: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. (DW: 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 DW: The Stolen Earth. In the real world, both Bumble Bees and Honey Bees are disappearing from the globe, a major concern.
 * When Donna begins to remember her adventures with the Doctor, she asks Wilfred why she can see a giant wasp, a reference to the Vespiform. (DW: The End of Time)
 * Sir Hugh mentions the Relief of Mafeking during the Boer Wars, at which the First Doctor was present. (DW: The Daleks' Master Plan, DW: The Invasion of Time)

Timeline

 * This story takes place after WC: The Lonely Computer
 * This story takes place before DW: Silence in the Library

Home video releases

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