The Invasion of Time, Part Three

Gallifrey, world of the Time Lords. The Doctor has claimed the presidency of the planet (on a technicality, following events in the previous season's The Deadly Assassin) and his behaviour is even more eccentric than usual in other ways, too: he refuses to speak to Leela and seems only to trust K9. But has he really betrayed Gallifrey to a Vardan invasion force? And is there a further power behind the Vardans?