The last stretch of the quest for the Key to Time brings the Doctor and Romana to the planet Atrios in the last days of a nuclear war with its neighboring planet Zeos.The Atrian ruler, Princess Astra, presses for peace, but the Marshal insists that his forces can win the war. Those near the Marshal fear that his judgment is suffering from the strain of command. He has taken to making most of his decisions following meditations before a strange black mirror. The Princess has disappeared, and the Marshal now rules unopposed. Have the Doctor and Romana arrived too late to complete their mission before Atrios is destroyed?When the Doctor learns that no living Astrian has ever met a Zeon, he proposes to travel to Zeos to negotate for peace and the return of Princess Astra, who seems to be linked in some way to the sixth segment. He finds the planet deserted. There are no living Zeons, but they left the computer running, and it's programmed for Mutually Assured Destruction.