Bill joins forces with some students from the university to find a house-share. At first, all their efforts are stymied, until they're invited to what seems like the perfect situation: a large old house with room enough for all of them, and affordable, to boot!
The Doctor helps Bill move her belongings in, and he seems concerned that something might be wrong at the house, so he sticks around without telling her at first.
Unfortunately, he was right to remain, because there's something seriously creepy at work in the house. The Caretaker seems able to come and go at will, and some of her house-mates begin to vanish.
The Doctor realizes that they are being absorbed into the house itself by extraterrestrial beetle-like creatures he dubs dryads. Bill discovers that the dryads are connected to the Caretaker's daughter Eliza... who appears to be completely wooden.
Only, she's not his daughter. She's actually his mother, transformed by the abilities of the dryads, powered by the lives of previous housemates. When Eliza realizes the truth of her condition, she offers her son the chance to find his own way.
After he refuses, she embraces him and they are absorbed by the dryads together, though she manages to restore Bill's housemates before the entire building collapses.
The Doctor returns to the Vault and shares dinner with his prisoner, to the consternation of Nardole.