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, but the station has been visited by Wirrn, an insect life form, which has laid its eggs in the solar stacks and absorbed the mind and body of one of the sleeping humans. Classic Doctor Who, from the fabled 70s Tom Baker era, and genuinely scary, despite some of the customary dodgy special effects and monster suits.