Sir Reginald Styles, a pivotal figure in an upcoming peace conference is attacked by armed guerillas, who then vanish. The Doctor and Jo wait in Styles' house and meet the guerillas when the return. They are from the future and plan to kill Styles before he can destroy the peace conference.
Jo accidentally gets transported into the future, and the Doctor follows her when the guerillas return. He finds that the Daleks invaded Earth following a nuclear war, that started after the peace conference was destroyed by a bomb apparently set by Styles.
The Doctor realises that the guerillas themselves destroyed the house, thus causing the war they tried to prevent. The Doctor and Jo return to the present and evacuate the peace conference moments before Shura, a guerilla left behind detonates a bomb in the cellar. The Daleks pursuing the Doctor are destroyed in the explosion.