The Doctor and Ace arrive on the earth colony of Terra Alpha, a colony that is under the misguided dictatorship of Helen A. On Terra Alpha, sadness and misery are capital crimes, and "killjoys' (anyone who does not adhere to these strict laws) are executed on the spot by Helen A's secret police, a motley crew of female assassins known only as "The Happiness Patrol". The Doctor and Ace make it their mission to put a stop to Helen A's harsh regime, but that's all easier said than done. What with The Happiness Patrol, undercover snoop Silas P, and Helen A's carnivorous little pet Fifi to contend with, they can do with all the help they can get, joining forces with Earl Sigma, rogue Happiness Patrol member Susan Q and the native inhabitants of Terra Alpha who were driven into the pipes and sewers of the colony. There is one threat that the Doctor hasn't banked on - Helen A's prized chief executioner, a crazed android and creator of killer confectionery.
