The Daleks are once again seeking their creator, Davros (last seen in 'Destiny of the Daleks'), to discover a cure for the Movellan virus. Mercenaries free Davros from his prison ship, but the Kaled scientist has other ideas, and soon a Dalek civil war is underway. On twentieth century Earth the Doctor (Peter Davison), Tegan and Turlough are caught up between the rival factions and the Earth rebels, but they are already part of a larger plan to destroy Gallifrey.