The TARDIS arrives on an apparently deserted and deactivated space station Nerva, otherwise known as the Ark, orbiting Earth in the far future. There the Doctor, Sarah and Harry discover the last survivors of the human race held in suspended animation, Earth having been evacuated thousands of years earlier when solar flares threatened to destroy all life.

The station has been visited by a Wirrn, an insect life form, which has laid its eggs in the solar stacks and absorbed the body and mind of one of the sleeping humans. The Doctor's reactivation of the station's systems causes the humans to start to revive. Their leader, nicknamed Noah, becomes infected by one of the emerging larvae and is slowly taken over.

The Doctor and his friends meanwhile gain the trust of the other humans, now led by a med-tech named Vira. Together they manage to lure the hatched Wirrn insects into a shuttle craft and then eject it into space.

In a final act of humanity, Noah - by this time fully transformed into a Wirrn - deliberately neglects to set the shuttle's stabilisers, causing it to explode.