On the planet Dido, the monstrous Koquillion terrorizes Bennett and Vicki, the only survivors of a crashed spaceship. When the TARDIS lands, the Doctor, Ian, and Barbara team up with Vicki to discover what Koquillion is hiding. (Originally broadcast in two parts.)

The Doctor and Ian reach the crashed spaceship and meet up with Barbara and Vicki, where the Doctor uncovers the truth about Koquillion.

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.

The TARDIS arrives on the Earth colony of Vulcan where the newly-regenerated Doctor assumes the identity of an Earth Examiner.

While the Doctor worries about the inert Daleks in the capsules, Lesterson makes plan to reactivate one of them.

The Doctor fails to convince the colonists of the danger posed by the Daleks and soon the other two are also reactivated.

With Polly being held prisoner by the rebels, the Doctor and Ben decide to investigate them, while Lesterson discovers the Daleks are reproducing.

The Doctor and Quinn attempt to escape in order to stop Bragen using the Daleks to take over the colony.

With Bragen having taken over, the Daleks trick the rebels into turning them loose and only the Doctor can stop them wiping out the entire colony.