Following on directly from the end of "The Parting of the Ways", the newly regenerated Tenth Doctor sets the TARDIS coordinates for the planet Barcelona (Tuesday, 6 p.m., October 5006) — where his predecessor had been planning to travel just before he regenerated – while Rose watches him suspiciously. He delightedly examines his new appearance (noting he has a slight weakness in his right wrist's dorsal tubercle, a full head of hair, sideburns, is slimmer, and has a mole between his shoulder blades), while asking her what she thinks of the way he looks.
Rose, confused and frightened, asks him who he is. When he tells her he is the Doctor, she does not believe him. Confused as to what she has just seen, she theorises that this stranger has replaced the Doctor by means of teleportation, or perhaps he is a Slitheen. She demands that he bring the Doctor back, and the Doctor tries to reassure her that it is him, telling her how they first met in the cellar at Henrik's, and the first word he ever said to her was "run".
Rose starts to believe him, and the Doctor leaps around the console happily, reminiscing about their other adventures, such as when they once had to hop for their lives. However, she is still unsettled and asks him if he can change back to his previous self. A deflated Doctor replies that he cannot, and asks her if she wants to leave. When Rose hesitates in her answer, he resets the TARDIS' coordinates for her council estate on Christmas Eve, offering her the choice to stay with her mother, Jackie, or continue her travels with him.
Suddenly, the Doctor suffers a form of seizure, expelling glowing energy from his mouth, and the TARDIS shudders as if in sympathetic response. The Doctor tells Rose that the regeneration is going wrong and starts to act maniacally, throwing switches on the TARDIS console and ranting about increasing speed, as the sounds of the Cloister Bell start ringing through the console room. As Rose hangs on to the console for dear life, the TARDIS heads for a crash landing on Christmas Eve.
