The one with crab people.
The TARDIS arrives at a human colony that seems happy enough but, as it turns out, the colonists are being mind-controlled by a race of giant crabs called the Macra. The Macra are using the humans to mine the toxic gases they need to survive.
Ben is affected by the Macra mind-control and starts to turn against the others, but the Doctor succeeds in breaking their hold and Ben destroys the gas-mining equipment, killing the Macra and freeing the colonists.
The Macra had a surprising cameo four decades later in the Tenth Doctor episode "Gridlock".
- Adult Fear: The story is an allegory for this. Turns out the 'happy life' society is set up to brainwash you into thinking you need - and enforcing to others - is to toil uselessly for decades in a job that benefits no-one except shadowy horrifying masters, slowly weakening and dying in order to afford the privilege of getting a week at Butlin's, listening to foul electronic pop music, and getting some new clothes and haircuts. We know Grand Control is right, and we must obey!!
- All There in the Manual: In A History of the Universe and A History, this story is arbitrarily dated to 2366 (it's in a time when Earth's colonies are remote, and the level of technology isn't very high).
- Big Bad: The Macra, specifically the one known as Control.
- Brainwashed: Attempted on the Doctor and his companions, but only ends up affecting Ben.
- Bedtime Brainwashing: The specific method of brainwashing attempted, mixed with a bit of pheromones for added effect.
- Cassandra Truth: Medok, the colony's last dissident, spends all his time trying to convince people that the Macra exist and being dismissed as a madman. By the time everyone knows the truth, he's dead.
- Crapsaccharine World: The story's setting is a society fashioned after a holiday camp.
- Deadly Gas: Deadly to humans at least, but essential nutrition for the Macra.
- Distressed Dude: Ben gets mind-controlled by the Macra and Polly has to snap him out of it.
- Extra-Strength Masquerade: The Macra so brainwashed the human colonists that an Extra-Strength Masquerade was relatively easy — until the Doctor turned up.
- Giant Enemy Crab: The Macra.
- Have a Gay Old Time: The Doctor comments "Well, this is gay" of the camp "happy worker" jingles broadcast over the Colony's speaker system.
- HeelFace Turn / Heel Realisation: The Pilot, after being convinced of the reality of the Macra.
- Masquerading As the Unseen: The Controller of the colony is only ever seen as a picture on the video screen. Jamie, eventually calls the colony's visible leaders on the fact that they don't know if it's actually the Controller sending out messages.
- Moral Dissonance:
- A number of fans have pointed out that this story depicts the Doctor collaborating with human colonisers to commit genocide on the indigenous sentient culture, who whatever they may have done to the interlopers didn't seem to pose an aggressive threat to the wider universe.
- A few lines in Gridlock claim the Macra had a good-sized empire before they devolved into mindless monsters.
- Also it's an odd bit of Fanon that the Macra were indigenous to the planet in the first place. It's not explicitly stated either way, but the fact they needed to terraform the planet, changing its atmosphere in order for them to survive there, suggests very strongly they were invaders from somewhere else. (It still could be they were there before the human colonists, though.)
- Muck Monster
- Names to Run Away from Really Fast: "The Controller" is not a reassuring name for the leader of a colony. Subverted as he's even more a slave than the rest of the colony.
- Oh, Crap!: The Macra freak out when the Doctor has the Pilot on his side and is about to use the gas from their lifeline to blow them all to Kingdom Come.
- Puppeteer Parasite
- Security Cling: See the page picture.
- Suspiciously Specific Denial: The quote above.
- Take That!:Ben: They got that bloke all over the place like a blinking politician.Pete: He is our controller. We are always pleased to see him. He brings us encouragement.Ben: Oh, he's not a politician then!
- Title Sequence Replacement: The first one in the series debuts here. It's also the first to use the Doctor's face.