Many thousands of years in the future, Earth becomes uninhabitable. Aboard space station Nerva, the remaining survivors of the human race drift amongst the stars in suspended animation. The Doctor, Sarah and Harry arrive to find vital systems have been sabotaged. As mankind begins to awake from its enforced deep sleep, the Doctor discovers that they are not alone... The Wirrn are an insect race that inhabit the cold depths of space. But they too have found the Ark and its sleeping occupants. Can the Doctor prevent the last of the human race from becoming the Wirrn's new source of food...?