The Tenth Doctor tells Martha that they are being pursued by the Family of Blood, who seek the Doctor’s Time Lord life force to prevent themselves from dying. He tells Martha that he must transform into a human to escape the Family’s detection until they die out, and gives her a list of instructions to follow. The Doctor turns himself into a human and transfers his Time Lord essence and memories into a fob watch that he asks Martha to guard.
They land on Earth in 1913. The Doctor has taken the persona of John Smith, a teacher at Farringham School for boys, and Martha acts as a maid at the school. John is quiet and timid, but faint memories of the Doctor slip through in his dreams. He catalogues the dreams in a book he has titled A Journal of Impossible Things. John keeps the fob watch on his mantle, believing it is a normal watch. John has also become infatuated with the school nurse, young widow Joan Redfern, and shares his journal with her. Martha is concerned, as the Doctor did not instruct her on what to do should he fall in love. Timothy Latimer, a young student at the school who has extrasensory perception, discovers the fob watch and bonds with it, seeing visions of the Doctor.
The Family of Blood track the Doctor to Earth, and cloak their ship with an invisibility shield to keep it hidden. The Family seek out humans to possess, and take the bodies of several people including one of the schoolboys, Jeremy Baines. They also animate scarecrows to use as their soldiers. When Timothy briefly opens the fob watch and experiences portions of the Doctor’s memories, the Family detects its presence at the school. Martha realises that the Family has found them, and attempts to retrieve the watch but cannot find it. John asks Joan to accompany him to the village dance that night, and she accepts. At the dance, Martha again tries to persuade John to become the Doctor by showing him elements of his past such as his sonic screwdriver. Now aware that John Smith is the Doctor, the Family interrupt the dance and confront him. They take Martha and Joan as hostages and give John a choice to either become a Time Lord again or watch his companions die.