Doctor Who: The Invasion of Time

The Doctor is behaving very oddly. After signing a treaty with enemy aliens, he returns to his home planet, Gallifrey and demands the Presidency of the High Council of Time Lords. His first act of rule is to banish Leela to the wastelands on pain of death. His second is to sabotage the planet's defence barriers, enabling full scale invasion. Why has the Doctor joined forces with the militaristic Vardans? Is he out for revenge against the Time Lords? Or does he have a hidden agenda? But his allies are also hiding a dark secret, for behind them is a familiar enemy waiting for the right moment to attack...