Helen A, ruler of colony Terra Alpha, is determined that happiness will prevail. And if any killjoys insist on being miserable, the fun guns of the Happiness Patrol will remove them; or they will vanish into the Kandy Kitchen, where the Kandy Man will deal with them. When the Doctor and Ace spend a night on the dark streets of Terra Alpha they have to keep a smile on their faces - or else! - while making contact with the native Pipe People and trying to convince the colonists that they can have too much of a good thing - even sweets and happiness.