On Earth, UNIT is called in to investigate a mysterious incident on a North Sea drilling platform. The Doctor believes something is afoot, and no sooner has the investigation begun when something even stranger takes hold: the Brigadier is starting to forget about UNIT's highest-profile prisoner. And he is not alone in his amnesia.
As the Sild invasions begins, the Doctor faces a terrible dilemma. To save the universe, he must save his arch-nemesis...the Master.