The Doctor returns to Gallifrey to become the President of the high council of time lords. But Leela finds the Doctor is acting out of character and she is proved correct, when it is revealed that the Doctor has betrayed his own people and is working for Invisible aliens known as the Vardans who have taken control of the citadel and The Doctor has banished members of the high council and Leela to the wasteland outside the Citadel. But the Doctor has his own agenda, he is planning to thwart the Vardans scheme and time-loop their planet. But what the Doctor doesn't know, is that the Vardans are working for the Sontarans who are the real masterminds and are preparing to invade Gallifrey.
