The Doctor, Steven and Dodo arrive on a giant spaceship millions of years in the future, transporting the last humans away from the doomed Earth.

With the Ark infected with a cold virus, the Doctor must convince the Guardians to trust him so he can find a cure.

The Doctor, Steven and Dodo have travelled 700 years into the future and find that the Ark is now under the control of the Monoids as it approaches Refusis.

The Monoids abandon the Ark, leaving Steven and the Guardians to die, but a mutiny in their ranks may give the Doctor a chance to bring about peace.