Christopher Eccleston portrays the Doctor in his ninth incarnation as the first season kicks off. A young woman named Rose Tyler (Billie Piper,) from England becomes the Doctor's companion after she helps him battle aliens in London. Together, Rose and the Doctor travel through time to the end of the Earth, Victorian England and Utah, where the two of them discover the last remaining Dalek, one of the Doctor's fiercest enemies. The first season comes to a close with the coming of the Doctor in his tenth incarnation.

In Season Two Scottish actor David Tennant picks up where Christopher Eccleston leaves off, and immediately he and Rose must save the world from another alien attack on Christmas. Together they reunite with familiar friends and continually face enemies. One of the biggest threats they face in Season Two is the return of the Doctor's old enemy, the Cybermen. By the end of the season, the Doctor and Rose have no choice but to say goodbye, and Rose is left behind in a parallel world with her mother, father and boyfriend Mickey.

Season three finds the Doctor feeling gun shy and heartbroken after leaving Rose behind, but he immediately picks up a new companion, Martha Jones (Freema Agyeman). At first even Martha notices that he's trying to recapture memories he shared with Rose, but in time they find their own adventures, including a trip to early twentieth century England where the Doctor gives up his Timelord self to save the world from an evil group that would use his essence to destroy everything. A second Timelord makes an appearance near the end of the season and holds Martha's family and the Doctor prisoner. It's up to Martha to save the world and the Doctor alone, and in the end she tells the Doctor she's had enough of time travel for awhile.

Season four begins in April 2008. The Doctor has a new companion, Donna Noble (Catherine Tate), and will travel with a couple old companions as well, continuing in his quest to keep the universe safe from disaster.