Doctor: Peter Capaldi (Twelfth Doctor)
Companions
Pearl Mackie (Bill Potts)
Matt Lucas (Nardole)
Others
David Suchet – The Landlord
Mariah Gale – Eliza
Mandeep Dhillon – Shireen
Colin Ryan – Harry
Ben Presley – Paul
Alice Hewkin – Felicity
Bart Suavek – Pavel
Sam Benjamin – Estate Agent
Tate Pitchie-Cooper – Young Landlord
Production
Directed by Bill Anderson
Written by Mike Bartlett
Produced by Nikki Wilson
Executive producer(s) Steven Moffat
Brian Minchin
Incidental music composer Murray Gold
Series Series 10
Length 44 minutes
Originally broadcast 6 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, and is troubled by the numerous noises the house makes.
Later, the Landlord mysteriously appears, ostensibly to check on the students. Suspicious of him, the Doctor tests him by asking who the Prime Minister is and names several, including Harriet Jones. The Landlord, however, evades the question and leaves. 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. Piano music plays from within as the Doctor enters to have dinner with the prisoner inside.
Knock Knock