[[caption-width-right:344: Oh crap, that astronaut stole all the color!]]
-> ''I don't know what came down in Recovery 7, but it certainly wasn't human!''
-->-- '''The Doctor'''
Mars Probe 7 disappeared with three astronauts on board several months ago, and now the British Space Agency's "Recovery 7" mission is in trouble. UNIT asks the Doctor to go and investigate.
Recovery 7 returns to earth, but the astronauts are kidnapped as soon as they land, and Liz notices that the capsule registers off the scale on her geiger counter. The "astronauts" are in fact aliens, dependent on radiation, who have swapped places with the humans for some reason.
The Doctor takes a solo flight in Recovery 7 and finds Mars Probe 7, still orbiting the red planet. A huge alien spaceship appears and the Doctor is taken on board. He finds the real astronauts, unharmed, and a very cross alien captain who threatens to destroy the Earth unless his "ambassadors" are released.
It turns out that General Carrington, in charge of the Space Agency, has kidnapped the ambassadors to provoke the aliens into attacking Earth, whereupon humanity will have to defend itself and destroy the aliens. Carrington had been on Mars Probe 6 which had encountered the aliens on Mars, where they accidentally killed his comrade by touching him.
The Doctor and UNIT stop him and arrange for a return of the ambassadors.
This story was recorded in colour, but the only surviving colour copies of episodes 2-7 are very poor quality ones recorded from broadcast by a US fan using early home video technology, from a Canadian station at the very edge of its transmitter area. In 2012, the story was released on DVD, restored to full colour by combining the video recordings with the chroma dot automated colour recovery process.
* BlastingItOutOfTheirHands: Done by the Brig on Reegan.
* DeadLineNews: A very unusual variation, with the good guys interrupting the villains.
* DoomedAppointment: When Lennox goes to UNIT with what he knows, he refuses to talk to anyone but the Brigadier because he's afraid anyone else might be working for General Carrington. By the time the Brigadier makes it back to UNIT headquarters, Lennox is dead.
* DragonInChief: Reegan is much more charismatic and threatening than the deranged and finally pathetic Carrington.
* DragonWithAnAgenda: Reegan's intention to use the aliens for ordinary crime once he gets a proper way of talking to them.
* EveryoneKnowsMorse: After being abducted by Reegan, the Doctor rigs an SOS broadcast signal to alert UNIT to his location. Sergeant Benton identifies the meaning of the signal, but remarks that nobody uses it any more.
* FailedASpotCheck: Carrington and the Doctor both completely fail to recognise each other when the Doctor steals the capsule back.
* FantasticRacism: Carrington's motivation.
* FirstContact: ''Doctor Who'' universe humanity's proper First Contact with an alien species not trying to invade, but gone badly wrong.
* GeneralRipper: Carrington
* GoodIsNotSoft[=/=]FamilyUnfriendlyViolence: The Brigadier is clearly seen to gun down several mooks in the course of this story, and has a very brutal fistfight with one during the final attack on Reegan's hideout.
* HeKnowsTooMuch: Lennox, the scientist keeping the aliens alive for Reegan, is killed when he decides to go to UNIT with what he knows.
* IDidWhatIHadToDo: General Carrington belives firmly in this.
* KnockoutGas: Reegan's kidnapping of the Doctor.
* NuclearNasty: The aliens "eat" hard radiation and can kill people or incinerate objects with a touch.
* PetRat: Reegan.
* PoisonousPerson: The aliens, thanks to their natural radioactivity.
* PragmaticVillainy: Reegan.
* RankUp: Corporal Benton is now a Sergeant.
* ReasonableAuthorityFigure: Cornish, the top man at the Space Centre.
* RevengeMyopia: Carrington has persuaded himself that the aliens are evil and must be destroyed because they accidentally killed one of his crewmates on an earlier mission.
* ShoutOut: The names on Reegan's chameleon van are shoutouts to Margot Heyhoe, the assistant floor manager, and Pauline Silcock, the director's assistant.
* StarfishLanguage: The aliens communicate by directly emitting and receiving electromagnetic waves rather than vocally.
* TheTeaser: The only classic [[Series/DoctorWho Doctor Who]] serial to use them for cliffhanger reprises, though they actually occur halfway through the title sequence, after the "Doctor Who" title appears but before the "The Ambassadors (p'tang) of Death" title.
* VulnerableConvoy: The convoy transporting the Recovery 7 capsule with the alien astronauts in it is attacked, and the truck carrying the capsule hijacked.
* WellIntentionedExtremist: Carrington is one of the ''extremely'' paranoid variety.
* WhatTheHellIsThatAccent: Professor Taltalian can't make up his mind whether he's French, German or something else entirely.
* YouHaveOutlivedYourUsefulness: Reegan letting his minions get fatally irradiated in the back of the van, and Taltalian's killing is halfway between this and HeKnowsTooMuch.
** A number of stunt performers are particularly clearly seen to die multiple times in the course of this story as soldiers or mooks, sometimes in the '''same episode'''. In later stories they would get better at disguising the stunt performers, or at least not showing close-ups of their faces.
** We first met Lennox on [[Recap/DoctorWhoS5E1TheTombOfTheCybermen Telos]].