Earth has lost contact with a probe sent to Mars seven months ago. A British space research center is overseeing a manned capsule, Recovery Seven, sent to retrieve the probe, with the Brigadier and UNIT providing added security. Back at UNIT headquarters, the Doctor is demonstrating the time-travel capabilities of the TARDIS to a skeptical Liz (although her skepticism dissipates when the Doctor accidentally sends her 15 seconds into the future), The Doctor’s attention is caught by a TV broadcast on the capsule’s launch and the fact that the broadcast includes a shrill noise the Doctor recognizes as being from an alien species. The Doctor heads off to the space center, where he finds that something responded to the alien broadcast from Buenos Aires—sorry, just kidding! After UNIT uses radars around the world to triangulate the signal, it naturally turns out to be from London.
After tracing the signal to a seemingly abandoned office building, UNIT gets into a gunfight with the men responsible for the signal. When the men are brought back to UNIT headquarters, they refuse to cooperate with the interrogation until the Doctor uses drill sergeant speak to get a kneejerk salute from one of the men, proving that this was a military operation. Even more suspiciously, it turns out that one of the scientists working at the space center, Dr. Taltalian, is working with whoever was giving orders to the soldiers in London and tries to take the recording of the alien signal from the Doctor and Liz at gunpoint, but the Doctor uses TARDIS technology to “transmogrify” the tape, keeping Dr. Taltalian distracted long enough for the Brigadier to show up and disarm him. Later the mystery only deepens when Rescue Seven reenters Earth orbit without any communications from the crew, who have apparently sealed the entrance to the capsule with themselves still inside. Before UNIT can retrieve it, two soldiers try to tow it away. The Doctor stops them by pretending to be a confused old man trying to get people to repair Bessie. When they try to push the car, their hands get stuck to Bessie thanks to an anti-theft forcefield with the Doctor cheerfully noting, “It will let you go…eventually.”
It turns out the soldiers and Dr. Taltalian are working for General Carrington, a former astronaut who now heads the UK’s Space Security Department with the blessing of a government minister, Sir James Quinian. It turns out that they are running their own investigation of what happened to Rescue Seven. While the Brigadier is appeased with this explanation, despite the violence deployed by Carrington’s agents, the Doctor insists on being taken to where the astronauts being kept. However, the three astronauts are abducted and the team of scientists investigating team are killed. Looking at their data, Liz notes to her surprise that Recovery Seven has been exposed to so much radiation no human could have survived being inside it.
It turns out that the three astronauts have been taken by Reegan, a London gangster who had been hired to take the astronauts. Reegan manages to have Liz abducted as well, enlisting her in his scheme to exploit the astronauts himself. Finding that the astronauts are really aliens who can be manipulated via mechanically generated signals and are dependent on radiation to survive but are immune to gunfire, Reegan schemes to use the aliens to commit conventional crimes in addition to his orders to keep the astronauts captive. Liz reluctantly agrees to help work on a device that would let him feed the aliens more elaborate commands, but in the meantime Reegan is still able to, under orders from his boss, use one of the aliens to assassinate Sir James Quinian before he can go to the Doctor and the Brigadier with “the whole truth”. After one of her escape attempts is foiled by Dr. Taltalian, who is also working for Reegan’s mystery boss, Liz manages to convince another scientist who is reluctantly working with Reegan, Tristan, to go to UNIT. Also, when Dr. Taltalian realizes that the Doctor suspects him, he goes to Reegan, who sets him up with a suitcase boobytrapped with the bomb. Of course, Dr. Taltalian thought the bomb was only meant for the Doctor, but Reegan meant it for both Dr. Taltalian and the Doctor, although it only manages to kill the former.
Meanwhile the Doctor deduces that the real three astronauts are actually still in orbit on an alien vessel and insists on flying the Rescue Seven capsule himself, glibly noting that he could handle “primitive” space flight. Reegan manages to sabotage the capsule and kill Tristan before he could meet the Brigadier for good measure, but nonetheless the Doctor does manage to arrive at where the astronauts are being kept. Inside the alien ship, they’re hallucinating that they’re on Earth, being kept under quarantine and watching a football match. An alien tells the Doctor that the three astronauts will be returned to Earth in exchange for the three ambassadors his people sent to Earth. If not, then Earth will be destroyed.
Returning to the planet, the Doctor is taken by Reegan and taken to his London labs and forced to help Liz finish the device for communicating with and controlling the aliens. However, Reegan’s schemes are spoiled when his boss, General Carrington, shows up. The Doctor learns that, when he was an astronaut, Carrington had an encounter with the aliens and witnessed them accidentally killing his fellow astronaut because of their radioactivity. Unable to handle the incident, Carrington became obsessed with the aliens and the threat they posed, to the point that he manipulated the aliens’ broadcasts to lure and trap their ambassadors. He is ready to take care of the Doctor and Liz personally, but Reegan convinces Carrington it’s more useful to let them finish the device and help expose the aliens as the threat Carrington knows they are. The Doctor does indeed finish the device, but he also uses it to broadcast a SOS that reaches UNIT, who storms Reegan’s London lab. However, the Brigadier and UNIT are unable to stop Carrington from taking over the space center in order to broadcast images of one of the ambassadors and a message urging the nations of the world to launch nuclear bombs against the alien ship. With the help of the ambassadors, UNIT storms the space center and gets past the soldiers still loyal to Carrington. The Brigadier orders Carrington’s arrest while the clearly unstable general only meekly insists that he was doing his “moral duty”.
Goon: Don’t try anything!
Liz: It’s alright. I won’t hurt you.
Unfortunately, this is one of those “Doctor Who” stories that’s more known for its troubled production than for the end product. Originally the episode was supposed to be a Second Doctor adventure with Jamie and Zoe, but it wasn’t used and instead, after going through multiple drafts, ended up as a Third Doctor story. And, sadly, it shows. The story is not only convoluted in a way that a plot that’s been reworked and reheated several times can be, but it bears the marks of one of the most common flaws of the show’s black-and-white era: bad pacing with a side of too many extraneous characters.
It’s a shame because there is an edge to this story that actually makes it feel like one of the Third Doctor adventures from this season. There’s the “humans and not the aliens might be the real bastards” angle, plenty of violence and death (along with a couple of effective action set pieces), the Doctor being arrogant and overbearing even to his allies, and the moral ambiguity surrounding some of the good guys. Still, though, even these elements are undercut. The writing makes it explicit that General Carrington is insane and has gone rogue. Even the fact that the government (albeit at Carrington’s prompting) sanctioned violence against UNIT isn’t cast under a sinister light; it’s actually laughed off.
At the very least, though, you get a fantastic Brigadier moment where he single-handedly escapes from two guards, so there’s that.