Parts: 7
Writers: David Whitaker and Trevor Ray (episode 1), Malcolm Hulke (episodes 2 - 7)
Airdates: 21 March 1970 - 2 May 1970
Production Code: CCC
Creature Feature: Ambassadors of Death
Special Guests: UNIT (the Brigadier, Benton)
Story: After seven and a half months of silence, Mars Probe 7 takes off from the surface of the red planet, startling the scientists on Earth who had believed the crew dead. As they are unable to establish communication, a recovery probe is sent to make contact with the crew and help them get back to Earth. But when the recovery probe docks with Mars Probe 7, and the astronaut on board enters the craft, a loud noise is heard over the radio...and then no more is heard from either craft.
As the Doctor, Liz and the Brigadier begin to investigate, it becomes clear that they are not the only group interested in the recovery probe. The Doctor determines that the noise on the radio was a message, but before they can even decode it, someone else has sent a reply. A raid on the reply's point of origin yields a single prisoner... who the Doctor swiftly discovers is military. When attempts are made to steal the recovery probe after its return to Earth, and then the astronauts inside it are smuggled out by men posing as UNIT members, the Doctor begins to suspect that a high-level conspiracy is involved.
Soon the Doctor's investigations begin to attract unwanted attention. After attempts are made on his life, and Liz is kidnapped and forced to work with the enemy, the Doctor and the Brigadier are unsure who can be trusted, or whether anything is as it seems. And when the Doctor realizes that the missing astronauts are actually still in orbit around the Earth, and their replacements from the capsule are in fact aliens who can kill with a touch, he has to wonder... have they really come to take over the Earth? Or are their true motives known only to the men who control them?