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
as The Doctor

Caroline John
as Liz Shaw

Nicholas Courtney
as Brigadier Lethbridge-Stewart

Ronald Allen
as Ralph Cornish

John Abineri
as Carrington

Robert Cawdron
as Taltalian

Michael Wisher
as John Wakefield

Cheryl Molineaux
as Miss Rutherford

Ray Armstrong
as Grey

Robert Robertson
as Collinson
CinemaSerf
The "Brigadier" (Nicholas Courtney) is having trouble contacting the returning Mars probe so drafts in the "Doctor" (Jon Pertwee) and "Miss Shaw" (Caroline John) to try and get to the bottom of the mystery. With a bit of help from the space centre director "Cornish" (Ronald Allen) he quickly conclud...