Doctor Who has returned Tegan to 1981. The Tardis has landed on a massive spaceship four days away from Earth. The spacecraft appears abandoned at first, but actually carries a force of frog-like aliens that may challenge the future of humankind on Earth. The spaceship 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. It is four to doomsday, and the Doctor has no time to lose.