The Doctor has returned Tegan to 1981 - the right time, but the wrong place. The TARDIS has landed on a massive spaceship four days away from Earth. It is the domain of Monarch, a scientific genius of almost god-like power. Monarch rules over three billion of his species, as well as a group of apparently immortal humans from Earth's history. What part has he played in Earth's past, and what are his plans for the future? Will he save humanity from itself, or be its destroyer? It's four to doomsday, and the Doctor has no time to lose finding out.