The TARDIS materialises in a human colony which is run like a giant holiday camp, where everybody is happy all round and fun and games are the order of the day. But a madman named Medok claims that the camp is stalked at night by hideous monsters, and as the Doctor learns more about the camp he realises that the people here risk their lives daily to mine a dangerously toxic natural gas – and yet nobody knows what it's for. His investigation attempt hits a snag when Ben is brainwashed by the colony's subliminal conditioning, and turns in his friends for acting against colony interests. In fact, the colony is under the control of the Macra, giant crab-like insects who are using the brainwashed humans to mine the gas they need to survive. It's up to the Doctor and his friends to prove the truth and free the colonists from a slavery they don't even know they're suffering.