When the Second Doctor, Polly, Ben and Jamie visit a human colony that appears to be one big holiday camp, they think they have come across a truly happy place. Yet a shadowy presence soon makes them realise that the surface contentment is carefully controlled. The colony's inhabitants have been brainwashed by giant, crab-like creatures — the Macra. Insidious propaganda, broadcast by the Controller, forces the humans to mine a gas that is essential for the Macra to survive, but fatal to them.