The planet Spiridon, the year 2540. The Doctor and Jo arrive on the trail of the Daleks, and find unexpected allies in the shape of a Thal suicide squad. But what has brought the Daleks here in the first place?

The Doctor is captured by the Daleks, while Jo is rescued by a friendly Spridon named Wester.

The Doctor and Codal escape from their cell and meet up with the Thal commandos who are trying to get into the Dalek base.

The Doctor and his friends escape from the Dalek base, prompting the Daleks to devise a plan to wipe them out.

The Doctor and his friends ambush two Daleks as part of their plan to get back into the city, but the Daleks are on the verge of unleashing their virus.

The Doctor and Sarah return to London in the present day only to find the city strangely deserted, while nearby UNIT are dealing with a series of monster appearances.

The Doctor is rescued from Finch by the Brigadier and Benton and tries to persuade them to stop Grover before millions of people are erased from history.