Doctor Who and the Invasion of Time (Doctor Who)

A traitor to the Time Lords? Can the Doctor really be in league with the evil Vardans, spearheading a treacherous invasion of his home planet, Gallifrey? Or is he playing a deadly double game, saving the Time Lords by appearing to betray them? But the Vardans themselves are only pawns in the game, and the Doctor faces an old and deadly enemy, as he battles to foil the Invasion of Time.