Wed, 10 May 2017
Bill and five students (Shireen, Pavel, Harry, Felicity and Paul), seeking to room together, take an offer by an elderly Landlord for a large mansion at very low cost, as long as they do not enter the tower. Bill gets the Doctor to help with her move. The Doctor insinuates himself among Bill's friends, to Bill's consternation, but is troubled by the numerous noises the house makes, including knocking back when the Doctor knocks on the walls.
As night falls, the knocking noises grow, and Bill's friends start to disappear. All the exits from the house are sealed tight, preventing any of them from leaving and separating the remaining ones. Bill and Shireen see Pavel half-absorbed in the wall, and the Landlord appears, striking a tuning fork which causes Pavel to disappear completely. The Doctor discovers the house's woodwork infested with insect-like creatures he calls Dryads. They are responsible for drawing the others into the woodwork prior to consuming them. The Doctor and Harry soon find evidence that every twenty years, a new set of students have been brought to the house to feed the Dryads. The Landlord arrives and admits that he needs the Dryads to keep his daughter Eliza alive in the tower.
The Doctor and Bill converge on the tower, finding Eliza's body is now made completely of wood. The Doctor determines that the Landlord is actually Eliza's son, a memory long forgotten. As a boy, he had brought his terminally-ill mother some dormant Dryads he found, unaware of their power. When they heard a high-pitched sound from her music box, they awoke and started turning Eliza to wood to stave off her illness. Since then, the Landlord has controlled the Dryads to keep Eliza well while signing on new tenants to become the Dryads' source of nourishment. Eliza is dismayed to find she has been "living" for so long without an actual life outside the house. Being able to control the Dryads, Eliza takes the Landlord into a hug, over his objections, and thanks the Doctor before having the Dryads consume them, while also reconstituting all of Bill's friends. The group escapes the house before it collapses in on itself.
Back at the university, the Doctor offers to take over watch of the vault from Nardole. The sounds of a piano can be heard coming from inside as the Doctor enters to have dinner with the prisoner inside.
When talking about the Time Lords, Bill asks "Do you wear robes and big hats", to which the Doctor replies "No, big collars mostly". This is a reference to the classic series when Time Lords did wear large collars with their robes, which were introduced in The Deadly Assassin. The Doctor also inadvertently mentions regeneration, but then quickly changes the subject.
The read-through for the second production block of the tenth series took place on 18 July 2016, and filming began on 1 August 2016, starting with the third episode of the series, "Thin Ice", and then the fourth episode, "Knock Knock". The episode was originally titled "The Haunted Hub".
The Fields House in Newport served as the mansion in this story; it had also been used for the Wester Drumlins house in the episode Blink. David Suchet stated he "completely freaked" when he realized on the third day of filming that his family had rented the exact same house the Christmas before for the holidays. 
In an earlier version of the script Harry, one of Bill's friends, was to be revealed as the grandson of a previous companion of the Fourth Doctor, Harry Sullivan, but this scene was later dropped from the final script.
Broadcast and reception
After the episode was first broadcast, people in the UK could view a binaural version of the episode on BBC iPlayer. The episode was watched by 4.32 million overnight, up a half-million viewers from the previous episode, "Thin Ice", and the highest-viewed story overnight since the series opener "The Pilot". It received an Appreciation Index score of 83.
"Knock Knock" received mixed reviews from critics, receiving criticism regarding the strength of the writing, but praise on the quality of the performances in the story, particularly Suchet's. "Knock Knock" holds a score of 85% on Rotten Tomatoes, the site's consensus reading "A bit of narrative weakness aside, "Knock Knock" is a wonderful mix of nostalgia, horror, humor, and monsters that culminates into a powerful and emotionally complex episode."