Knock Knock ( TV story ) 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 . 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 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 came 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 on 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 revebererate 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 lound 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 saying 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 pulls 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 an giant insect ; like a woodlouse with glowing antenna . He speculates this is likely an alien being , a wood nympth , 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 ' s 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 an 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 a 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 . Story notes . Production errors . " to be added " Home video releases . DVD releases . " to be added " Blu - ray releases . " to be added " Digital releases . " to be added " External links . " to be added "