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.
as The Doctor
as Liz Shaw
as Brigadier Lethbridge-Stewart
as Ralph Cornish
as Carrington
as Taltalian
as John Wakefield
as Miss Rutherford
as Grey
as Collinson
as Van Lyden / Alien Ambassador
as Quinlan
as Control Room Assistant
as Dobson
as Corporal Champion
as UNIT Sergeant
as Reegan
as Lennox
as Heldorf
as Alien Ambassador / Lefee
as Alien Ambassador / Michaels
as UNIT Soldier
as Masters
as Flynn
as Sergeant Benton
as Private Parker
as Control Room Assistant
as Control Room Assistant
as Technician
as Alien Space Captain
as Aliens' Voices
as Private Johnson
