The TARDIS arrives on the planet Terra Alpha after The Doctor hears of some disturbing rumours of something evil on the planet. As The Doctor and Ace explore they become unsettled by the planet’s unnaturally happy society. Cheerful music plays everywhere and the planet’s secret police force, the Happiness Patrol, roam the streets wearing bright pink and purple uniforms, while they hunt down and kill so-called ‘Killjoys’. The Doctor and Ace soon encounter Trevor Sigma, an official galactic censor, who is visiting Terra Alpha to discover why so many of the population have disappeared.
When some members of the Happiness Patrol arrive and start painting the TARDIS pink The Doctor and Ace then get themselves arrested. This happens when The Doctor points out that he and Ace are without official off-worlder badges. The Happiness Patrol’s leader, Daisy K, has them arrested and they are taken to a holding zone controlled by Priscilla P. There they meet Harold V, who used to be Harold F, and whose brother has been sentenced to death for a display of public grief. Harold V tells The Doctor and Ace that the planet is ruled over by the vicious and egotistical Helen A, who is obsessed with eliminating unhappiness. He also tells them that the penalty for those found guilty of unhappiness is death in a stream of molten candy prepared by Helen A's executioner, the robotic Kandy Man.
After Harold V is killed, by an electric shock from a fruit machine, The Doctor and Ace manage to escape from the holding zone in a motorised go-cart. The Doctor though first has to disarm a bomb that has been planted on it. Ace though is re-arrested but she manages to escape when she is assisted by Susan Q, a disaffected Happiness Patrol member. The Doctor meanwhile narrowly avoids being re-arrested by Happiness Patrol spy Silas P, who is knocked unconscious by a man named Earl Sigma - a holidaying medical student, who has got stuck on Terra Alpha, and who has been stirring unrest by playing the Blues on his harmonica. When Silas P comes to, he is killed by the arrival of a Happiness Patrol who discover that he is unhappy.
Earl Sigma and The Doctor head for the Kandy Kitchen where they are captured by the Kandy Man, who plans to use them in its experiments. They escape when The Doctor verbally confuses the Kandy Man and gets it to spill some lemonade causing the Kandy Man to become stuck firmly to the floor. The Doctor and Earl Sigma then escape into a network of pipes and tunnels under the kitchen where they meet up with a group of Pipe People, the original inhabitants of Terra Alpha. On learning from the Kandy Man what has happened Helen sends Fifi, her carnivorous pet Stigorax - a vicious dog-like creature -, into the pipes to flush them out.
Ace is re-arrested once more and taken to a holding zone with Susan Q. She is rescued by one of the Pipe People, but Susan Q is taken off for a ‘routine disappearance’. Ace finds herself being chased by Fifi down the tunnels but she uses a can of her nitro-9 to blow up the pipe behind her but then falls down another pipe. She emerges to join Susan Q on the execution platform - and so in danger of being killed by a deadly flow of boiling candy.
|Helen A and Fifi
The Doctor also manages to escape from the pipes and he again meets Trevor Sigma, who he persuades to take him to see Helen A. They arrive at her office and she tells Trevor Sigma that she has managed the population down by seventeen percent. The Doctor then takes a fire extinguisher from the inner office and a lemonade siphon from the outer office. The Doctor then returns to the Kandy Kitchen where the Kandy Man is still stuck firm to the lemonade covered floor. The Doctor unsticks him with a blast from the fire extinguisher in return for the Kandy Man’s agreement to divert a flow of boiling candy destined for the execution platform. With Ace and Susan Q thus saved, The Doctor uses the lemonade to re-attach the Kandy Man to the floor.
When the execution fails Helen A is furious and so enrols Ace and Susan Q to appear in the late show at the Forum, where the penalty for non-entertainment is death. The Doctor and Earl Sigma rescue them both by arranging for some factory workers, that he encountered earlier, to demonstrate in the forum square. He then persuades a couple of snipers to stand down. Helen A, undaunted, orders the Happiness Patrol to wipe out the demonstrators.
Ace and the Happiness Patrol arrive at the forum square to be greeted by The Doctor, who seems ecstatically happy to see them – which means that the Happiness Patrol cannot touch him. The demonstrators also appear happy, as do Ace and Susan Q. This leaves only the Happiness Patrol themselves who are unhappy. Priscilla P therefore arrests them making Helen A even more furious.
|Painting the TARDIS Pink
Fifi, having survived Ace’s earlier nitro-9 blast in the tunnel, and after being tended by Helen A, is sent back down into the pipes to wipe out the ‘vermin’. There The Doctor, Ace, Susan Q, Earl and the Pipe People find themselves being pursued through the pipes by Fifi. They arrive at a section of pipe encrusted with sugar and The Doctor gets Earl Sigma to play an ‘A flat’ on his harmonica in the hope that this will create a resonance and cause the sugar to collapse. This fails, so Earl Sigma tries a ‘C’ instead. But this also fails. However, as Fifi approaches them and howls, the crystallised sugar comes crashing down on the creature.
The Doctor and Ace then head for the Kandy Kitchen, where they threaten the Kandy Man with a red hot poker and the heat from the oven. The Kandy Man escapes into the pipes but is engulfed and dissolved by a flow of his own fondant surprise started by the Pipe People. Only a metal skeleton and some sweet remains emerge onto the execution platform.
While a revolution takes full effect outside the palace walls Helen A realises that she has lost control of the population and so starts to pack in preparation to leave Terra Alpha. As she is dong so a radio announcer reports that one hundred and twelve factories have fallen to the rebels. Helen A’s private shuttle then takes off, but she is not on board as it has been commandeered by Gilbert M, who constructed the Kandy Man and acted as its assistant, and Joseph C, Helen A’s husband. After they bid her farewell, via a video screen, Helen A makes her way through the city and meets The Doctor who tries to teach her about the true nature of happiness, which can only be understood if counter-balanced by sadness. Helen A at first sneers at The Doctor as she is still convinced that she was right and that unhappiness and love were unnecessary. But as she plans to leave, on a scheduled flight, Helen A finally realises that happiness is nothing without the contrast of sadness when she discovers Fifi, now injured and dying. This causes her to finally feel some sadness of her own when she breaks down in tears at the loss of her pet.
With Helen A’s rule over, and the revolution complete, Priscilla P and Daisy K are put to work repainting the city as The Doctor and Ace bid farewell to Susan Q and Earl Sigma. On discovering that the TARDIS has been mostly repainted, back to its original blue, Ace picks up a paint pot to finish off the job.