The Doctor and Romana arrive on the planet Atrios looking for the crucial final segment. Atrios and the nearby planet Zeos are waging an interplanetary nuclear war, which has already killed most of the Atrions. Despite this, the Marshal of Atrios is preparing for a fanatical last strike that will utterly destroy Zeos, against the wishes of the Peace movement headed by the Princess Astra! The Doctor realises that the Marshal is being hypnotically controlled, and has to confront the evil Shadow, a servant of the Black Guardian!