The TARDIS lands on a seemingly deserted space station in the distant future only to discover that they are not alone. While the station's crew rests in cryogenic sleep their Ark has been invaded by a parasitic insect race, the Wirrn.

The TARDIS falls through a CVE into E-Space and arrives on the planet Alzarius. There the inhabitants of a crashed Starliner and a group of young rebels called the Outlers, led by a boy named Varsh and including his brother Adric, are being terrorised by a race of Marshmen who emerge from the marshes at a time known as Mistfall.

The Doctor visits the Shadow's lair.

The truth about the sixth segment is discovered, but Armageddon is only seconds away.