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 "Gridlock".