Knock Knock (TV story)

 was the fourth episode of series 10 of Doctor Who.

This was the first Doctor Who story to be made available with a binaural 360 soundscape, released on iPlayer.

The episode expands the Doctor and Bill's relationship briefly when he tells her about the Time Lords, the alien race he belongs to and quickly mentions regeneration to her, but doesn't explain what regeneration actually is to her. This was the first explicit, on-screen reference to the Twelfth Doctor's then-upcoming regeneration. The fact is referenced various times throughout the rest of the series.

It also makes a reference back to the Doctor's granddaughter Susan Foreman, as Bill passes the Doctor off as her grandfather and refers to him as such. She has also told her friends about him; although, the Doctor doesn't seem to have much of a problem with it.

The Vault is also implied to not just have the mass it appears to have, instead, being dimensionally transcendental. The prisoner inside is heard playing a piano the Doctor gave them. Nardole also suggests that the prisoner is aware of when the Doctor is starting to neglect looking after them.

Synopsis
Bill and her friends rent an old house to live in, but the floorboards and the walls are creaking, and there's a creepy Landlord lurking around. Can the Twelfth Doctor save the day as creatures come crawling out of the wood?

Plot
Bill opens her door to find her friend Shireen and their four new housemates outside. Shireen introduces them as Felicity, Harry, Pavel and Paul. They head to the estate agents to find a place to rent together as student accommodation. However, they have a restricted budget and the agent that helps them shows them around places that are less than satisfactory. As the group leave the estate agents on the verge of giving up, they are met by an elderly man named John, who asks them if they are looking for a place to live. He leads them to a magnificent old house, which they assume will cost them a lot to rent. However, the Landlord assures them that rent will be cheap, but specifies that the tower is off limits.

The group explores the house, and all except Bill are enthusiastic about the size of the place. Bill remains suspicious about the cheapness of the rent, suspecting that there is a catch. Pavel asks to move in that night as his student halls are kicking him out, to which the Landlord happily agrees, getting everyone to sign a contract. That night, Pavel sets up his record player in his room and starts it playing; shortly afterwards he is surprised by something horrifying.

The next day, Bill is stacking up her belongings in boxes in her room. After everything is piled in one corner, she looks at her phone and counts down. The TARDIS materialises over the boxes, which end up inside. The Doctor opens the door and expresses his surprise that Bill has so few possessions.

Bill enters the TARDIS, suggesting that the Doctor hire it out as a removal service. The Doctor is offended at the idea, thinking that removals is beneath a Time Lord. A little puzzled, Bill asks if 'Time Lord' is the Doctor's job description. He explains that it is the name of his species. Bill finds this hilarious, remarking how posh it sounds, to which the Doctor replies that it was the pompous attitude of his people that convinced him to run away. Bill asks if Time Lords wear robes and big hats. The Doctor replies that Time Lords tend to wear big collars instead of hats.

When asked if he needs the address to find the house, the Doctor scoffs; however, he then relents and tells Bill to type it in so the TARDIS can land there. She asks if he sleeps in the TARDIS bedroom which she found. The Doctor tells her that Time Lords don't need sleep, except after big meals and regenerations. Bill is confused by his mysterious words, but the Doctor is evasive when questioned, saying it's enough to know that he's a Time Lord.

The TARDIS lands outside of the house, and the Doctor expresses his surprise that Bill and her friends can afford to rent it. Bill thanks the Doctor for helping her, telling him he can leave once her boxes are out of the TARDIS. However, the Doctor has noticed something off; he tests the breeze before offering to help Bill move her things inside. Bill is reluctant, seemingly worried that the Doctor's presence will be embarrassing and raise too many awkward questions, but he has already started carrying a box to the house.

Inside, the Doctor is greeted by Shireen, who knows about him. Bill passes the Doctor off as her grandfather, despite his protests. Felicity, Paul and Harry join them, having also been told who the Doctor is. Bill tries to get rid of the Doctor, who seems to relent and leaves the house.

That night, as the friends are gathered in the lounge eating takeaway meals, they discuss odd things that they have noticed about the house. Felicity complains that there is no mobile phone signal whilst Harry reports hearing tapping, like footsteps, when he was unpacking. Paul and Shireen laugh this off and tease Felicity until a loud banging from the kitchen makes everyone freeze. The group squabble over who should investigate until Bill takes the lead and the others follow. As the clattering continues from the pantry, Bill summons her courage and opens the door, surprising the Doctor, who has been poking around the room with his sonic screwdriver. Bill is exasperated, saying she thought he had gone home. The Doctor ignores the comment, telling the kids that there is no washer and dryer, no central heating and that the outlets will not take their devices; it seems nothing in the house has been updated since the 1940s. Bill takes the Doctor to one side and tries to persuade him that there is no mystery involved, but the Doctor is unconvinced, mentioning that the creaking they heard in the trees outside could not have been caused by the wind because there wasn't any. He unsuccessfully tries to convince the group to rent a different house. Paul decides to call the Landlord about updating the house but Shireen reminds him that there is no signal.

Everyone heads into the sitting room, where the Landlord is unexpectedly waiting. He claims to have come by to check on things when he notices the Doctor, and Bill tells him the cover story about him being her grandfather. The Landlord guesses that he's helping with relocating Bill and wistfully states that there is no worse feeling than leaving children to fend for themselves. The Doctor thinks about these words, remembering when he did so. The Landlord says that he's lucky to have a daughter that is still under his protection, before asking if everything is satisfactory. He gets a barrage of complaints about the house from the students and says that he will do what he can to address these concerns; though he seems untroubled, almost amused by their problems. When Harry asks how they get into the tower the kindly facade of the Landlord drops and he becomes sinister. As he leaves, the Doctor follows him and asks him who the current Prime Minister is. When the Landlord cannot answer, it confirms his suspicion that there is something amiss. Remembering that they need a washing machine, Shireen opens the front door to follow him, but the Landlord is nowhere to be seen.

Bill tries getting the Doctor to leave, by suggesting he sleep in "the car"; however, the Doctor instead decides to stay up with Felicity and Harry to listen to some music. Seeing she can't win, Bill decides to head to bed, followed by Shireen and Paul; before she goes, the Doctor suggests checking on Pavel, who hasn't been seen all day. As they head to their rooms, Paul tries to ask Bill out, but she tells him she's gay. He takes the news well, and politely backs off, noting that he never had a chance. He heads into his room, teasing Shireen for her fear of the house by making creaking noises on the floorboards. Once he shuts his door, Bill asks why Shireen didn't take a room next to her, as they had planned. She is defensive over the question, leading Bill to conclude that she fancies Paul. At that moment, Paul is heard screaming in terror and the girls begin to doubt that this was part of his joke. When Shireen knocks on his door, answering knocks reveberate all around the hallway and doors begin to slam shut; Bill and Shireen flee upstairs.

Downstairs, Felicity and Harry ask the Doctor what he thinks of the music playing on her phone. He notes that it reminds him of Quincy Jones, whom he once filled in for. The bass player for Quincy's band turned out to be a killer Voc Robot in disguise; what was worse, he couldn't even play. He then directs their attention to the front door, which has sealed shut. They hear loud banging sounds from the kitchen and follow the noise to see the shutters slamming themselves closed. Felicity panics, and, grabbing the shutters to keep them from closing, escapes through the window. She hastily dials the police, but is standing close to a nearby tree; it absorbs her, as she screams in horror.

Hearing the violin music still playing on a loop in Pavel's room, Bill and Shireen enter to find Pavel partially absorbed into the wall. Shireen goes to turn off the record, but Pavel blinks rapidly to communicate that they shouldn't. The Landlord suddenly appears and says that "hope is its own form of cruelty", lifts the stylus from the record; Pavel is absorbed completely. He tells them that their friend is now at peace, one with the house. The girls are horrified and realise that they will be next. When Bill challenges him, the Landlord replies that everyone must pay their dues, and since the six signed a contract, it is time to pay.

The two race out of the room and down the hall to a bookcase; deciding it must open a way into the tower, Bill and Shireen pull books off until they find a switch disguised as one and pull it. A way opens into the tower and they run up into what appears to be an old bedroom. Shireen finds a music box, and opens it; a haunting melody plays prompting her to shut it. A woman's voice calls out, asking if her father has come back. Bill and Shireen believe this is the voice of the Landlord's daughter, who says she hasn't had company for some time. She comes out of hiding, revealing herself to be completely made of wood. She introduces herself as Eliza.

Downstairs, Harry is panicking because the house managed to somehow get Felicity when she was outside. The Doctor tells him not to be scared, as it isn't useful in their current situation. Believing something is living in the wood, the Doctor begins knocking on the wood to get a response. He presses hard on a cupboard door and a gap opens to release a giant insect; like a woodlouse with glowing antenna. He speculates this is likely an alien being, a wood nymph, or a dryad possibly. The Doctor tells Harry to get a box so he can examine it, however, they have to abandon this plan when a whole swarm pours out of the wood. The Doctor pulls Harry into the cupboard, revealing that it is a lift.

The lift takes them down to the basement, where the Doctor and Harry discover the possessions of past groups of six that the Landlord lured to the house. Each group of boxes has a contract, identical to the one Bill's friends have signed, which are dated 1997, 1977, and 1957; every twenty years. It becomes clear that the Landlord has been luring young people to the house to feed them to the insects. Hearing footsteps, they head outside of the room, where the Landlord meets them.

The Doctor confronts the Landlord with the names of his previous victims, whose fate he seems to regret, but when the Doctor asks why he feeds people to the insects, he insists it was necessary. He explains that his daughter was dying until she was saved by the insects and that she must survive. Harry panics and attempts to escape, but his foot is trapped in the wooden staircase and the Landlord summons the lice with his tuning fork. They swarm over Harry and absorb him. The Landlord turns his attention to the Doctor, who backs away from the insects, and in desperation, suggests that as a doctor he can help the man's daughter.

In the tower, Eliza tries reaching out to shake Shireen's hand. However, Shireen backs away in fear. She asks what's the matter. Shireen tries to leave calmly, but after stamping on what she believes to be a cockroach, is consumed by a swarm of the insects. Eliza glows with light as she absorbs Shireen's life energy. Moments later, the Doctor is lead into the room by the Landlord, who explains that he has brought a doctor to help her. Bill explains that Shireen was taken by the lice; the Doctor confirms that Harry suffered the same fate.

The Doctor examines the daughter, coming to the conclusion that the Landlord brought the odd insects to her when she was sick to amuse her. However, the insects respond to high pitched sounds, such as the sound of her music box; so when he checked on her the next morning, he found that her wooden transformation was beginning, but her illness was cured. Bill then notes the oddity of it; why would a father spend time looking for insects in the garden?

The Doctor congratulates Bill on her quick thinking, remarking that he forgot that humans don't live long, so if the father preserved his daughter seventy years ago, he couldn't still be alive. Eliza becomes confused, as the Doctor explains that her memories have decayed due to the transformation process and the passage of time. The Landlord is actually her son, who did whatever he could to keep his mother alive. Prompted by the Doctor, Eliza realises that her son has committed atrocities throughout his life just to keep her alive in an inhuman condition. She asks her son to let her die, but he refuses and summons the insects to consume the Doctor and Bill. With the Doctor's encouragement, Eliza realises that she can control the insects, and thanking the Doctor for his help, has the insects devour herself and her son.

Without Eliza to act as the linchpin, the house begins crumbling. As they are about to run from the room, Bill sees Shireen remerging from a swarm of the insects on the floor. Bill is overjoyed and hugs her friend, as the Doctor explains that Eliza has restored and released the most recent victims. Everyone runs outside, just in time to watch the house crumble. His work done, the Doctor tells the group to go back to the estate agents; he heads for the TARDIS as Bill and the others gape at the sight of the destroyed house.

Back at the Vault, Nardole is doing diagnostics. The Doctor arrives with bags of takeout food, telling Nardole he can have the rest of the night off. When Nardole turns to leave, he hears Fur Elise play on piano in the Vault; he notes that the Doctor doesn't know when to quit interfering until it comes back to haunt him. The Doctor firmly tells Nardole "Good night", sending him off.

The Doctor approaches the Vault, asking if the prisoner would like some Mexican food; he also has a new story. However, the prisoner doesn't respond until the Doctor mentions that people get eaten in his story; to this, the prisoner plays Pop Goes the Weasel. Taking this as confirmation, the Doctor smiles and unlocks the Vault to join his prisoner for dinner.

Cast

 * The Doctor - Peter Capaldi
 * Bill - Pearl Mackie
 * Nardole - Matt Lucas
 * The Landlord - David Suchet
 * Eliza - Mariah Gale
 * Shireen - Mandeep Dhillon
 * Felicity - Alice Hewkin
 * Paul - Ben Presley
 * Harry - Colin Ryan
 * Pavel - Bart Suakvek
 * Estate Agent - Sam Benjamin
 * Young Landlord - Tate Pitchie-Cooper

Culture

 * Bill makes reference to Indiana Jones.
 * Shireen says "I'm a celebrity, get me out of here", a reference to the reality TV series. She also refers to the building as a "freaky Scooby-Doo house".
 * Paul says that a freshers' party is being held in the park.
 * Fireworks are used at the party.

Music

 * behind the Vault can play the piano.
 * Pavel has a vinyl record, DVD players and speakers.
 * Bill has a whole playlist on Spotify with Little Mix on her phone. The playlist includes the song Black Magic.
 * One of the tenants from 1977 owned the David Bowie album Heroes.
 * Pavel has Bach: Sonata #1 by Itzhak Perlman playing in G Minor for Solo Violin, BWV 1001 - 1. Adagio.
 * The figure behind the Vault plays Für Elise by Beethoven and Pop Goes The Weasel, a famous nursery rhyme, on a piano.
 * The song heard at the start when Bill and her friends are looking for a house is "Weird People" by Little Mix.

Locations

 * When Felicity bemoans the lack of phone reception, Shireen says she needed a landline and Felicity jokingly asks "What is this, Scotland?".
 * A house on Mill Lane, Bristol, is shown on the estate agent's computer.
 * The park is the location of a freshers' party.
 * Shireen has once been to Morocco.

Businesses

 * A shop named The Fabric Mill is seen.
 * A cement factory can be found next to one of the houses that the students check out.

Technology

 * The house has no washing machines.
 * The plugs can't take the students' phones.
 * The house has an old-fashioned television set.
 * The hob is from the 30s.
 * Pavel plays a vinyl record using a USB turntable.
 * The Doctor uses the sonic screwdriver as a light source.
 * Eliza has a music box.
 * There's a typewriter in the basement.

Science

 * The Doctor finds an oil burning heater.

Species

 * 11 Cardinal Road has skulls from gazelles hanging on the walls.
 * Paul and Shireen jokes to Felicity that the house possibly has a "doll, that's come to life" or a "massive, freaky spider", to which Bill says it's probably just a mouse, and Felicity indirectly says she has a fear of mice.
 * Bill asks if there's a cat in the house. The Landlord denies this.
 * Time Lords need to sleep after regeneration.
 * At first, the Doctor suggests that the walls could be infested with wood nymphs or tree spirits.
 * Upon discovering the Dryads the Doctor says he expected a gaseous creature or a microscopic one.
 * The Doctor calls the woodlice Dryads, though he later reverts to just calling them lice.
 * The bass player in Quincy Jones' band turned out to be a Klarj Neon Death Voc-Bot.
 * Shireen thinks the Dryads are cockroaches.
 * The Doctor mentions flies.

Foods and beverages

 * The house has a picture of a bowl of pears.
 * There's a book on Farmhouse Cooking in the kitchen.
 * The students eat Chinese. The Doctor loves Chinese.
 * The Doctor (noisily) eats a krupuk.
 * The Doctor brought Mexican food for himself and the prisoner in the Vault.

People

 * Bill has a teddy bear.
 * Harry's grandfather and his boyfriend once got arrested for trying to steal a bit of the Great Wall of China.
 * Bill hangs up a photo of her mum.
 * Asking the Landlord who is the Prime Minister, the Doctor lists Margaret Thatcher, Harriet Jones, Harold Wilson and Anthony Eden as candidates. (This was a test to see if he was up on current events; an alien or time travelers would likely know the wrong one.)
 * The Landlord has a tuning fork.
 * The Doctor calls Harry "Simon" and Felicity "Florence".
 * The Doctor once sat in on a recording session for Quincy Jones when the bass player (who couldn't play anyway) was revealed to be a Klarj Neon Death Voc-Bot.
 * Felicity is Musophobic and Claustrophobic.
 * Felicity tried to call the police.
 * Shireen wanted to call an ambulance.
 * A tenancy agreement from 2 September 1997 was signed by Jake Christie, Annie Wren, Jonathan Frost, Sarah Tiller, Mark Hopethorne and Carl Richards. There is a further agreement from April and signed in May 1977 and one from 1957.

Organisations

 * Bill and her friends go to Redcliffe Lettings to find a new house to live in.
 * Redcliffe Lettings' phone number is 01174 960313.

The Doctor

 * The Doctor offhandedly mentions regeneration to Bill; he avoids elaborating on it.
 * The Doctor still can't tell people's ages.
 * He shares stories with the Vault's prisoner, only if they're gruesome enough.

TARDIS

 * Bill suggests the Doctor make the TARDIS a removal service.

Story notes

 * A working title for this episode was The Haunted Hub.
 * The read-through for Knock Knock took place on 18 July 2016, and filming took place in August.
 * David Suchet said in The Fan Show: The Aftershow that he used a real life experience of being covered in woodlice for the scene where the Dryads come crawling all over the Landlord.
 * In an interview for Doctor Who Magazine #512, writer Mike Bartlett indicated that Harry was intended to be the grandson of Fourth Doctor companion Harry Sullivan, but the reference was cut. A possible remnant of this has Harry recognising the Doctor's name and calling him a "legend", despite the episode establishing that Bill is not yet closely acquainted with her new flatmates; another is the fact that Harry is partnered up with the Doctor for part of the episode. There is also a dialogue reference made to Harry's grandfather and his boyfriend stealing a piece of the Great Wall of China, however it is not clear whether this refers to Harry Sullivan or another grandparent.
 * "Weird People" by Little Mix is playing extra-diegetically in the scenes where Bill and her friends are looking for a new house.
 * Shireen refers to 11 Cardinal Road as "a freaky Scooby-Doo house". The episode was partly filmed on location on the same property used for Wester Drumlins in TV: Blink - in which a character also referred to the place as "Scooby-Doo's House".
 * Not really noticed by everyone, but everything that Bill and her friends brought to that house either ended up buried or destroyed when it fell apart.

Ratings

 * 4.32 million (UK overnight figures)
 * 5.73 million (UK final)

Filming locations

 * Fields Park Road, Newport, South Wales

Production errors

 * When the Doctor offers to take Bill's things to her room using the TARDIS, the door is closed, but Bill goes to retrieve some more of her things from inside and closes the door behind her.

Continuity

 * The Doctor mentions Prime Minister Harriet Jones, who ascended after aiding his ninth incarnation in defeating the Slitheen. (TV: Aliens of London/World War Three) His tenth incarnation, however, would set in motion her deposition after she ordered the destruction of the retreating Sycorax. (TV: The Christmas Invasion) She would help save the world one final time against the Daleks. (TV: The Stolen Earth)
 * The TARDIS materialises around Bill's possessions. It had done so previously around passengers. (TV: The Parting of the Ways, Blink, The Runaway Bride, Dinosaurs on a Spaceship, Hell Bent)
 * The Doctor mentions that he ran away from his people. (TV: The Name of the Doctor)
 * The Doctor says he loves Chinese. (TV: The Power of Three)
 * Bill says there are bits of her life that the Doctor is not in. Amy Pond and Rory Williams used to have a double-life, (TV: The Power of Three) and so did Clara Oswald. (TV: The Day of the Doctor)
 * When Bill says they should get the Doctor, Shireen asks if he's gonna give a lecture. (TV: The Pilot)
 * The Doctor remarks on the Time Lords' dress sense, having done so earlier in his life. (TV: Time Crash, Journey to the Centre of the TARDIS, Nightmare in Silver, Hell Bent) He makes note of the high collars (as seen in TV: The Deadly Assassin, et al).
 * The Doctor reveals he's a Time Lord. (TV: The War Games et al., Doctor Who, The End of the World et al.) Upon learning this, Bill scoffs at the Doctor being part of the aristocracy (TV: The Caretaker).
 * Regeneration is mentioned. (TV: The Tenth Planet/The Power of the Daleks, The War Games, Planet of the Spiders/Robot, Logopolis/Castrovalva, Destiny of the Daleks, The Caves of Androzani, Time and the Rani, Doctor Who, The Night of the Doctor, The Day of the Doctor, The Parting of the Ways, Utopia, The Stolen Earth/Journey's End, The End of Time, Day of the Moon, Let's Kill Hitler, The Time of the Doctor, Hell Bent) However, the Doctor glosses over the topic, similar to how his tenth incarnation did such with Martha Jones in regards to his race. (TV: Gridlock)
 * The Doctor had previously faced creatures who absorbed people into walls. (TV: Flatline)
 * Bill introduces the Doctor as her grandfather. The Doctor once travelled with his granddaughter Susan Foreman, who called him grandfather. (TV: An Unearthly Child - The Dalek Invasion of Earth, The Five Doctors)
 * The Doctor takes offence at being described as Bill's grandfather, suggesting father be more accurate given his appearance. (TV: The Caretaker)
 * Bill's sexuality is mentioned as she informs Paul, after he makes romantic overtures, that he isn't her type and she prefers girls. (TV: The Pilot)
 * Bill puts a picture of her mother, which was taken by the Doctor, on the wall. (TV: The Pilot)
 * One of the flat mates theorise dolls could be creeping around upstairs. The Doctor encountered a house of dolls in his eleventh incarnation. (TV: Night Terrors)
 * The Doctor uses the sonic screwdriver as a light source. (TV: Thin Ice)
 * The Doctor has met people made of wood before. (TV: The End of the World, The Doctor, the Widow and the Wardrobe)
 * The Vault is tended to by Nardole once more, the Doctor revealing a piano is inside. (TV: The Pilot, Thin Ice)
 * In an attempt to convince herself that there's nothing strange about the house, Bill remarks that "There's no living puddles or weird robots, big fish..." (TV: The Pilot, Smile, Thin Ice)
 * The Doctor claims sleep is for tortoises, (TV: The Talons of Weng-Chiang) as he doesn't sleep much himself, (TV: Good Night) usually only after regeneration (TV: Spearhead from Space, Robot, Castrovalva, Time and the Rani, The Christmas Invasion, Deep Breath) or after a big lunch. (TV: The Two Doctors)
 * The Doctor refuses to "high-five" Harry. (TV: Time Crash, Under the Lake)
 * Nardole says the prisoner has gotten a little restive lately. (TV: Thin Ice)

DVD releases
This episode was released as part of the Series 10 DVD box set on 13 November 2017.

Blu-ray releases
to be added

Digital releases
to be added