[[caption-width-right:350:[[LeaningOnTheFourthWall Yeah, well you two are no prizes either!]]]]

->''"This is an emergency! Control must be believed and obeyed! No-one in the colony believes in Macra! There is no such thing as Macra! Macra do not exist! There are no Macra!"''
-->-- '''Control''' (protesting too much)

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 [[Recap/DoctorWhoS29E3Gridlock "Gridlock"]].


* BigBad: The Macra, specifically the one known as Control.
* {{Brainwashed}}: Attempted on the Doctor and his companions, but only ends up affecting Ben.
** BedtimeBrainwashing: The specific method of brainwashing attempted, mixed with a bit of pheromones for added effect.
* CassandraTruth: 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.
* DeadlyGas: Deadly to humans at least, but essential nutrition for the Macra.
* ExtraStrengthMasquerade: The Macra so brainwashed the human colonists that an ExtraStrengthMasquerade was relatively easy -- until the Doctor turned up.
* GiantEnemyCrab: The Macra.
* HaveAGayOldTime: The Doctor comments "Well, this is gay" of the camp "happy worker" jingles broadcast over the Colony's speaker system.
* HeelFaceTurn / HeelRealisation: The Pilot, after being convinced of the reality of the Macra.
* MoralDissonance: 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 ''DoctorWhoS29E3Gridlock'' claim the Macra had a good-sized empire before they devolved into mindless monsters.
* MuckMonster
* NamesToRunAwayFromReallyFast: "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.
* PuppeteerParasite
* SecurityCling: See the page picture.
* SuspiciouslySpecificDenial: The quote above.
* TakeThat:
-->'''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!
* TitleSequenceReplacement: The first one in the series debuts here. It's also the first to use the Doctor's face.