In this four part adventure, the Doctor, Harry and Sarah accidentally arrive on an artificial satellite where the survivors of Earth lie in cryogenic suspension waiting to begin a new life. But the Doctor soon discovers that there has been a sinister intrusion... A gigantic insect has infiltrated the human ark. It is dead, but it was Queen of the Wirrn, sworn enemies of the Earth — and her egg sac was full. Something is terribly wrong and when the crew begin to awaken it soon becomes clear that some of them are not entirely what they seem... The Doctor must find out how to destroy the Wirrn larvae before they absorb the last of the human race and become an invincible swarm. But this involves a desperate gamble with his own life — a gamble he must take if he is to save mankind.