From a ruined world at the end of time, the vicious Sild make preparations to conquer the past and rewrite history. But to do it they will need to enslave an intellect greater than their own... On Earth, UNIT is called in to examine a mysterious incident on a North Sea drilling platform. They've hardly begun, though, when something even stranger takes hold: The Brigadier and others are starting to forget about UNIT's highest-profile prisoner. As the Sild invasion begins, the Doctor faces a terrible dilemma. To save the universe, he must save his arch-nemesis... The Master.