UNIT are placed in charge of security at a crucial Peace Conference, guarding Sir Reginald Styles, the diplomat with the best chance of averting a war between the Chinese and Russians. A mysterious guerilla appears, attempts to kill Styles and then disappears with Jo and the Doctor - into a terrifying future Earth ruled by the Daleks and their brutish slaves, the Ogrons. The guerillas explain that the Daleks overran the Earth after a nuclear war back in the 20th century - caused by Styles. They believe the war and subsequent invasion can be averted by killing Styles. But are they right?