The British Space Program under Professor Ralph Cornish (Ronald Allen) launches the Recovery Seven Probe to find the missing Mars Probe and two missing astronauts. The pilot of Recovery Seven, Van Lyden (Ric Felgate), makes contact, but is silenced by an unearthly sound. The Doctor (Jon Pertwee) and Liz Shaw (Caroline John) travel to the Space Centre to investigate. The Doctor thinks the sound is a coded message, and it appears to have elicited a reply from a warehouse nearby. Brigadier Lethbridge Stewart (Nicholas Courtney) and UNIT troops respond. There is a gun battle with troops commanded by General Charles Carrington (John Abineri).
Recovery Seven returns to Earth. While UNIT is transporting it to the Space Centre, it is attacked and hijacked by Carrington’s troops. The Doctor locates it, but only after the astronauts have been removed. Carrington is installed by the Minister for Technology, Sir James Quinlan (Dallas Cavell) as the new head of Space Security. He says the astronauts are infected with contagious radiation. Carrington takes the Doctor to meet the astronauts, but they have been abducted by Reegan (William Dysart) and the soldiers and scientists guarding them are dead. The Doctor and Liz realize that humans could not survive what the astronauts went through, that they are alien imposters and the real astronauts are still in orbit. Liz is kidnapped and forced to help Carrington’s scientist Professor Lennox (Cyril Shaps). The aliens (who look rather mysterious and scary behind their fogged-up space helmets) are sent to the Space Centre to kill Sir James. They are spreading deadly radiation. Liz helps Lennox escape, but he is killed by Reegan.
Cornish wants to launch another spacecraft to rescue the real astronauts. The Doctor pilots the launch himself, but an enormous spacecraft swallows his ship and that of the stranded astronauts, who believe they are on Earth. An alien tells the Doctor that the men are being held until their peaceful ambassadors are returned, but the Earth will be destroyed if they are not. The Doctor returns to Earth, is kidnapped by Reegan, and General Carrington is revealed as the mastermind. He has met with aliens before, traumatically, and his mind is broken. He wants the world to turn against the aliens. He leaves for the Space Centre, where he intends to reveal the aliens as evil and get the Earth’s military powers to destroy their ship in orbit. The Doctor calls for help, UNIT rescues him and Liz, arrests Reegan and his men, then the Brigadier arrests Carrington. The Doctor arranges for the aliens to be returned in exchange for the astronauts.
This story, greatly inspired by the Quatermass films, had been written in 1968 for the Second Doctor but was postponed, and UNIT was added to make it work as a Third Doctor story. As a result, the first episode, which involved the apparent loss of a space-probe, aired on the same day as the troubled launch of Apollo 13. The effort to turn Doctor Who into a Bond-like action series was well-served by the BBC’s hiring of a stunt-man company named Havoc, as in “Cry Havoc, let slip the dogs of war.” UNIT was involved in a shoot-out in nearly every episode, and I particularly liked the company’s helicopter, numbered G-AWFL, which I took to mean God-awful. The Doctor’s classic car, Bessie, wore the license plate WHO-1, which was borrowed from a dentist, who later complained that he was never thanked. Since it was not actually registered to Bessie, every time she was filmed on a street or a highway, she was breaking the law.