25 January 1975
Tom Baker (Doctor Who), Elisabeth Sladen (Sarah Jane Smith), Ian Marter (Harry Sullivan)
Gladys Spencer, Peter Tuddenham (Voices) , Wendy Williams (Vira) [2-4], Kenton Moore (Noah) [2-4], Christopher Master (Libri) , Richardson Morgan (Rogin) [3-4], John Gregg (Lycett) , Gladys Spencer (High Minister’s Voice) , Stuart Fell [3-4], Nick Hobbs  (Wirrn Operators).
|Written by||Robert Holmes|
|Directed by||Rodney Bennett|
|Produced by||Philip Hinchcliffe|
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 invasion… 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.
It is only mentioned once by Vira but Noah’s name is Lazer, Noah being a joke on his role on Nerva.