The time travellers visit a human colony that appears to be a happy place run along the lines of an enormous holiday camp but has in fact been infiltrated and taken over by a race of giant crab-like creatures - the Macra.
The brainwashed inhabitants are forced to mine a gas toxic to themselves but vital for their oppressors' survival. Ben at one point comes under the Macra's malign influence and turns against his friends.
He eventually regains his senses, however, and under the Doctor's guidance destroys the gas pumping equipment, thus killing the Macra and restoring the colony's freedom.
COMMENT Spoliers ahead