The one with the giant talking earplugs.
The TARDIS lands on a desert planet and the travelers find two crashed spaceships and their occupants; the hideous Rills (and their cute robots, nicknamed "Chumblies" by Vicki) and the beautiful female Drahvins. The two have had a space battle above the planet and crash-landed, but the planet is disintegrating.
The Rills turn out to be the good guys innocent explorers who were attacked without reason by the warlike Drahvins, mindless clones led by a tyrannical matriarch.
With escape imperative and their own ship a complete write-off, the Drahvins try to force the Doctor, Vicki and Steven to help them capture the Rills' ship, but the Doctor instead powers up the Rills' ship from the TARDIS, allowing them to escape and then departs himself, leaving the Drahvins to their deaths.
This story is thought to be one of the last to be wiped before the episode junkings were stopped in 1978, though there's some dispute about that. All four episodes (besides about eight minutes of Episode 1) were lost until 2011, when Episode 3 (Air Lock) was returned to the BBC. Reconstructions note can be watched here. A slightly condensed official reconstruction consisting of all surviving footage with stills, new animations and the original soundtrack was released as an extra on the 2013 DVD release of "The Aztecs".
Peter Capaldi brought the story quite a lot of attention when he claimed the Chumblies were his favorite villain from the show upon being cast as the Doctor, and responded to everyone's confusion with "And you call yourselves Doctor Who fans?note "
- Absolute Xenophobe: Maaga and by implication all the other upper-caste Drahvins.
- Ace Custom: The Drahvins are so wedded to their caste system that they self-destructively only give their leader caste the good weapons.
- Ambiguously Human: It is unclear whether the Drahvins are human-looking aliens or a human colony that went in a really extreme cultural direction. Maaga refers to herself and her people as "human" or "human beings" several times, but this could just be translation convention given that it's in the context of "sentient beings deserving of life".
- Bad Boss: Seems to be required by Drahvin law and/or custom.
- Beauty = Goodness: Defied.
- Beauty Is Bad: The Drahvins are the villains, while the Rills turn out to be good.
- Bee People: The Drahvins are trying to make their society into this.
- Brainless Beauty: The barely sentient Drahvin clones.
- Call-Back: The astral map makes an appearance.
- Clone Army: Only Drahvin leaders are born the normal way, and the rest are cloned. Maaga is of the 'leader' caste, while the other Drahvins are clones who have been genetically modified to be almost mindless worker drones.
- Continuity Nod: The Doctor and Vicki refer to their trip to Xeros.
- Continuous Decompression: Enforced by the need to patch damage sustained by the film print of Air Lock.
- Dark Is Not Evil: The Rill look pretty ugly (from the photo we have), but they're a Perfect Pacifist People who desperately want the Drahvins to stop shooting at them.
- Dumb Blonde: Most of the Drahvins, but justified (see above).
- Earth-Shattering Kaboom: The planet's fate.
- Evil Cannot Comprehend Good: The Drahvins can't understand self-sacrifice.
- Exact Time to Failure
- Fantastic Caste System: The Drahvins have a caste system based on social insects, of female leaders, female workers, and a few males kept solely for the female leaders to sexually reproduce.
- Flashback: A Rill recounts the truth about the murdered Drahvin to Vicki. The events are shown in flashback from his point of view.
- Gender Rarity Value: We only ever meet the female Drahvins and their leader, Maaga, is initially confused when Steven asks about their men. When she does understand her response is an offhand remark that they keep as many males as they need and simply kill the rest.
- The Great Repair: Both the Rills and the Drahvins must repair their spacecraft in order to escape before the planet explodes. The Drahvins are incapable of doing so, and plan to kill the Rills and take their craft. The Rills offer to peacefully take them back into space, but peaceful co-existence isn't an option for the belligerent and warlike Drahvins.
- Lady Land: On the planet Drahvin, women dominate the planet. A few men are kept, presumably for breeding, while the rest are killed. Oddly, Maaga doesn't immediately understand when Steven asks her if all the inhabitants of her planet are women, perhaps demonstrating how utterly insignificant and out of mind the men of Drahvin are.
- No Indoor Voice: The Rill always speaks in a deep, booming voice.
- Oh, Crap!
- One-Gender Race: The Drahvins are a seemingly all-female race. They use the few males only for breeding. Their commanders are naturally born and their footsoldiers are clones.
- Pig Man: Deliberately subverted: the Rills look porcine but are deeply cultured and refined.
- Rubber-Forehead Aliens: The Drahvins look like blond women except with dark spotted patterns in place of eyebrows.
- Space Battle: The incident that set off the events of the story is a standoff in space between the Rill and Drahvin spacecraft where each shot the other down. It isn't shown onscreen, and it happened before the Doctor, Steven and Vicki arrive. The fight between the Drahvin and Rill spacecraft is related first by Maaga to the Doctor, and then later on by the Rills, who set the record straight.
- Starfish Aliens: The Rills.
- Stupid Evil: The Drahvins are determined to steal the Rills' spacecraft and leave them to die despite the fact that the Rills have offered to take them with them.
- Surrounded by Idiots: Justified example as Maaga has a couple of speeches lamenting the fact that her superiors forced her to crew her ship with a bunch of unintelligent, unimaginative worker caste who she rightly considers too untalented for space exploration.
- Telepathic Spacemen: The Rills don't speak in the conventional sense. They communicate telepathically, relaying their thoughts as speech through the Chumblies.
- Trailers Always Spoil: In a classic example, the big twist (that the beautiful Drahvins were the villains of the piece) was given away in a BBC press release published by the Daily Mail. Although in the story, Maaga acts pretty unsubtly villainous from the moment that the regular characters first meet her.
- You Have Outlived Your Usefulness: Maaga executes her own badly wounded soldier, then blames it on the Rills.