The one that got a trailer.
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 is investigating, and the Doctor (still miffed with the Brig by the Silurians incident) and Liz sort of drop in.
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.
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 but brainwashed, 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, after which the Doctor promptly swans off back to UNIT base, satisfied to leave the paperwork to other people.
As you might've guessed, this was where that electronic scream-like sound you hear at the cliffhanger or close of most Doctor Who episodes debuted.
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.
The episode also marks the return of companion Corporal John Benton (now a Sergeant), last seen in "The Invasion".
- Blasting It Out of Their Hands: Done by the Brig on Reegan.
- Continuity Nod: The Doctor is still angry at the Brigadier for blowing up the Silurians.
- Dead Line News: A very unusual variation, with the good guys interrupting the villains.
- Doomed Appointment: 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.
- Dragon-in-Chief: Reegan is much more charismatic and threatening than the deranged and finally pathetic Carrington.
- Dragon with an Agenda: Reegan's intention to use the aliens for ordinary crime once he gets a proper way of talking to them.
- Everyone Knows Morse: 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.
- Failed a Spot Check: Carrington and the Doctor both completely fail to recognise each other when the Doctor steals the capsule back.
- Fantastic Racism: Carrington's motivation.
- First Contact Faux Pas: Doctor Who universe humanity's proper First Contact with an alien species not trying to invade, but gone badly wrong.
- Flapping Cheeks: Happens to the Doctor when he leaves Earth by conventional rocket.
- General Ripper: Carrington. His actions are prompted by xenophobia driven by his own encounter with the alien beings when he piloted Mars Probe Six some years earlier. His co-pilot, Jim Daniels, was killed on contact with the aliens and Carrington signed the treaty with the aliens to lure three of their number to Earth, where he hoped he could unveil their real agenda of alien invasion.
- Good Is Not Soft/Family-Unfriendly Violence: 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.
- Graceful Loser: Carrington in the end.
- He Knows Too Much: Lennox, the scientist keeping the aliens alive for Reegan, is killed when he decides to go to UNIT with what he knows.
- I Did What I Had to Do: General Carrington belives firmly in this.
- Innocent Aliens
- Insane Admiral: General Carrington. A tragic example, he saw his crewmates unintentionally killed by the Ambassadors. This drove him insane, and he honestly believed he was acting in the Earth's best interest by destroying the Ambassadors, unknowing of the dire consequences this would unleash.
- Knockout Gas: Reegan's kidnapping of the Doctor.
- Nuclear Nasty: The aliens "eat" hard radiation and can kill people or incinerate objects with a touch.
- Pet Rat: Reegan.
- Poisonous Person: The aliens, thanks to their natural radioactivity.
- Poor Communication Kills: A justified example, due to the fact that the aliens communicate via radio waves instead of sound. No one realized this until the Doctor correctly identifies a seemingly random burst of noise as a signal and subsequently makes an attempt to decipher it. Since no one can understand the aliens, their radioactive nature and unintentionally deadly physical touch makes them easy to misunderstand as murderous invaders, when, in fact, all they originally wanted to do was make diplomatic contact.
- Pragmatic Villainy: Reegan.
- Rank Up: Corporal Benton is now a Sergeant.
- Reasonable Authority Figure: Cornish, the top man at the Space Centre. He starts out rather annoyed at the way the Doctor bulls his way into the situation (which, let's be honest, would be fairly annoying), but after a little while sees that he really knows his stuff and learns how to work with him.
- Revenge Myopia: 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.
- Shout-Out: The names on Reegan's chameleon van are shoutouts to Margot Heyhoe, the assistant floor manager, and Pauline Silcock, the director's assistant.
- Starfish Language: The aliens communicate by directly emitting and receiving electromagnetic waves rather than vocally.
- Stop Trick: Used in one of the first scenes, in which the Doctor is attempting to repair the TARDIS's Time Vector Generator, which accidentally sends both himself and Liz forward in time by a few seconds. First Liz disappears, then the Doctor, then Liz reappears, and the Doctor reappears. It proves to be a mild Chekhov's Gun when the Doctor does something similar with an important tape recording to keep it out of an enemy's hands.
- The Teaser: The only classic 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.
- Tragic Bigot: Carrington developed his hatred of the aliens after one of them accidentally killed his crewmate.
- Vulnerable Convoy: The convoy transporting the Recovery 7 capsule with the alien astronauts in it is attacked, and the truck carrying the capsule hijacked.
- Well-Intentioned Extremist: Carrington is one of the extremely paranoid variety. During his backstory, he misinterpreted the accidental killing of his partner as a murder; fast-forward to the present-day, and he is now determined to stir up a global effort to annihilate the aliens.
- What the Hell Is That Accent?: Professor Taltalian can't make up his mind whether he's French, German or something else entirely.
- Whole Plot Reference: The plot owes a great deal to The Quatermass Experiment (returning astronauts being aliens).
- The X of Y
- You Have Outlived Your Usefulness: Reegan letting his minions get fatally irradiated in the back of the van, and Taltalian's killing is halfway between this and He Knows Too Much.