The Doctor (David Tennant) is being pursued by the Family of Blood, which wants his Time Lord Lifeforce to preserve their lives. He gives Martha (Freema Agyemen) instructions and then becomes a human being, transferring his life essence into a fob watch, which she will guard.

The TARDIS lands on Earth in 1913. He becomes John Smith, a teacher at Farringham School for Boys, and Martha becomes a maid at the school. John Smith is mild-mannered and quiet, but his Time Lord memories appear in his dreams, which he writes down in a diary called A Journal of Impossible Things. The watch is on his mantle. He becomes infatuated with the school nurse Joan Redfern (Jessica Hynes) and shares his journal with her. Martha is concerned; she has no instructions on what to do if he falls in love. A young student with extra-sensory perception named Timothy Latimer (Thomas Sangster) becomes fascinated with the images he gets from the watch.

The Family of Blood track the Doctor to Earth in an invisible ship. They take the bodies of several people, including a schoolboy named Jeremy Baines (Harry Lloyd). They animate scarecrows to use as soldiers. When Timothy opens the watch, they sense it. Martha tries to retrieve the watch but cannot find it. John Smith asks Joan to the dance and she accepts. At the dance, Martha tries to bring the Doctor back by jogging his memory with such things as the sonic screwdriver. The family interrupts the dance and confronts the Doctor. They take Martha and Joan hostage and tell John to become a Time Lord or see them die. To be continued.

The story was written by Paul Cornell, based on his Doctor Who novel, and was well-received. It was nominated for a Hugo along with the second part, The Family of Blood. Previous Doctors appear as memories of terrifying and merciless Time Lords. John Smith says his parents were Sydney and Verity—that is, creator of Doctor Who Sydney Newman and first producer Verity Lambert. He thinks his birthplace of Gallifrey is somewhere in Ireland. John Smith is rather good at Cricket, picked up from the Fifth Doctor Peter Davison. The aliens’ guns are alive. There is a creature inside and pulling the trigger makes it scream, which disintegrates the target. The Family of Blood is simply vile. The actors Brodie Sangster and Harry Lloyd were both on Game of Thrones. Harry Lloyd is the great great great grandson of Charles Dickens. Martha is central to the story and Freema Agyeman is great. Tennant creates a fully formed other character in John Smith.