Astronauts rescued from a mission to mars are kidnapped when they arrive back on earth. They seem to have been irradiated, and can now kill people merely by touching them. But the Doctor discovers the astronauts have been replaced with Alien ambassadors who are being forced to behave in a hostile manner. With The Doctor traveling to the alien mother ship, and Liz kidnapped, The Brigadier is left wondering who to trust.
Jon Pertwee
The Doctor
Caroline John
Liz Shaw
Nicholas Courtney
Brigadier Lethbridge-Stewart
Ronald Allen
Ralph Cornish
John Abineri
Carrington
Robert Cawdron
Taltalian
Michael Wisher
John Wakefield
Cheryl Molineaux
Miss Rutherford
