Giant crustacean-like creatures which infiltrated an Earth colony, using subliminal messages to hide their presence. While the colony slept, the Macra absorbed the planet's natural gases deep underground, which resembled the gases on their own planet.
They conditioned the colonists to work for them, refining the gases, and ignoring their presence - they even managed to stop the colonists seeing the murder of their own Controller.
The conditioning failed on some colonists, and the Doctor cut off the supply of the gas to the Macra. Victory was clinched when Ben Jackson blew the gas pumps up, freeing the colony from domination by the Macra.
The Macra made a surprise return in Series 3, 40 years after their debut in Gridlock with David Tennant..
The Macra in Gridlock (2007)
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