While the Brigadier investigates a loss of contact with Mars Probe 7, the Doctor realises signals coming from the probe are of extra-terrestrial origin.
Quest roles:
Nicholas Courtney(Brig. Lethbridge-Stewart), Terry Walsh(Doran (uncredited)), John Abineri(Ranquin), Ronald Allen(Ralph Cornish), Michael Wisher(Magrik), Robert Cawdron(Taltalian), Ric Felgate(Astronaut), Cheryl Molineaux(Miss Rutherford), Ray Armstrong(Grey), Robert Robertson(Collinson)