The TARDIS arrives on the planet Terra Alpha, where the Seventh Doctor and Ace discover a society in which sadness is against the law - a law enforced zealously by the brightly uniformed Happiness Patrol. The planet is ruled by Helen A with the aid of her companion, Joseph C, and her carnivorous pet Stigorax, Fifi. Joseph C and Gilbert M escape in a shuttle, while the Kandy Man is destroyed and Fifi killed. Helen A finally realises the hard way that happiness is nothing without the contrast of sadness.