-> ''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''' protests 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 slightly surprising cameo four decades later in the Tenth Doctor episode "[[Recap/DoctorWhoNSS3E3Gridlock Gridlock]]".
* 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
* MissingEpisode: All four episodes are missing. Only a few censor clips and some off-air clips exist.
* 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.
* 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.
* TheOtherDarrin: Chicki was played by different actresses in part 1 and part 4.
* SecurityCling: See the page picture.
* SuspiciouslySpecificDenial: The quote above.
-->'''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.