The Doctor and Ace arrive on the Earth colony planet Terra Alpha. The Doctor has heard disturbing rumours of something evil on the planet and intends to get to the bottom of them. They meet Trevor Sigma (John Normington), an official from Galactic Centre who is on the planet conducting a census of the populace. 

The Doctor intends to start his investigation by getting himself arrested, so when some members of the planet’s pink-garbed Happiness Patrol (Argie Alaime, Olwyn Atkinson, Heather Downham, June Easther, Selina Gilbert, Julie Lawrence, Carole Mudie) arrive and start painting the TARDIS pink, he points out that he and Ace are without official off-worlder badges. The leader, Daisy K (Georgina Hale), has them arrested. They are taken to a holding zone controlled by Priscilla P (Rachel Bell), where they meet Harold V (Tim Barker) who used to be Harold F and whose brother (Cy Town) has been sentenced to death for a display of public grief. Harold V tells the Doctor and Ace of Helen A (Sheila Hancock), the ruler of Terra Alpha, and her executioner the Kandy Man (David John Pope). 

Harold V is killed by an electric shock from a fruit machine and the Doctor and Ace escape from the holding zone in a motorised go-cart after the Doctor first disarms a bomb that has been planted on it. 

Ace is re-arrested, but is allowed to escape by Susan Q (Lesley Dunlop), a disaffected Happiness Patrol member. The Doctor meanwhile narrowly avoids being re-arrested by Happiness Patrol spy Silas P (Jonathan Burn), who is knocked unconscious by a man named Earl Sigma (Richard D Sharp). When Silas P comes to, he is killed by a Happiness Patrol as he is unhappy. 

Earl explains that he is a holidaying medical student who has got stuck on Terra Alpha. He also plays blues music on a mouth organ. He 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, which sticks it firmly to the floor. 

The Doctor and Earl run into a network of pipes and tunnels under the Kitchen where they meet up with a group of Pipe People (Wences: Phillip Neve; Wulfric: Ryan Freedman; other Pipe People: Bilent Hassan, Charles Martin, Steve Martin, Lee Pearse), the original inhabitants of Terra Alpha. On learning from the Kandy Man what has happened, Helen sends Fifi, her 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’. 

The Doctor emerges from the pipes and again meets Trevor Sigma, whom he persuades to take him to see Helen A. They arrive at her office and she tells Trevor that she has managed the population down by seventeen percent. The Doctor takes a fire extinguisher from the inner office and a lemonade siphon from the outer. 

Fifi chases Ace and a Pipe Person down the tunnels. Ace uses a can of nitro-9 to blow up the pipe behind her but then falls down another pipe and emerges to join Susan Q on the execution platform. The Pipe Person scurries off and escapes. 

The Doctor returns to the Kandy Kitchen – on the way persuading a sniper to lay down his weapon, thus saving some striking factory workers from death – where the Kandy Man is still stuck firm. 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. 

Helen A, furious that the execution failed, enrols Ace to appear at the forum – her death is to take place in five minutes’ time. The Doctor tells Earl to bring the demonstrating factory workers to the forum square. He then persuades a couple of snipers (Mark Carroll, Steve Swinscoe) to stand down. Helen A, undaunted, orders the Happiness Patrol to wipe out the demonstrators. 

Fifi survived the nitro-9 blast in the tunnel and, tended by Helen A, has now recovered completely. Helen A sends it back down into the pipes to wipe out the ‘vermin’ therein. 

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 Patrol cannot touch him. The demonstrators also appear happy, as do Ace and Susan Q. This leaves only the Patrol themselves who are unhappy. Priscilla P therefore arrests the Patrol. Helen A is furious and orders her to let them go. 

The Doctor, Ace, Susan, Earl and the Pipe People are chased through the pipes by Fifi. They arrive at a section of pipe encrusted with sugar and the Doctor gets Earl to play an ‘A flat’ on his mouth organ in the hope that this will create a resonance and cause the sugar to collapse. This fails, so Earl tries a ‘C’ instead. Fifi approaches closer and howls, which brings the sugar crashing down on the creature. 

A radio announcer (Anne Hulley) reports that the Nirvana Sugar Beet plant has fallen to the guards and drones. Helen A is calm, reasoning that this is just one factory out of the thousand she has set up. She confers with Daisy K about who to send to quell the riots. Daisy K suggests Priscilla P. Priscilla P, however, is tied up and held at gunpoint by Susan Q in the holding zone. 

The Doctor and Ace 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 boiling candy started by the Pipe People. Only a metal skeleton and some sweet remains emerge onto the execution platform. 

Helen A is packing to leave Terra Alpha when the radio announcer reports that one hundred and twelve factories have fallen to the rebels. Helen A’s private shuttle takes off, but she is not on board. Instead it is piloted by Gilbert M (Harold Innocent), who constructed the Kandy Man and acted as its assistant, and Joseph C (Ronald Frazer), Helen A’s husband. They bid her farewell via a video screen. 

Helen A makes her way through the city and meets the Doctor. She is still convinced that she was right and that unhappiness and love were unnecessary. She plans to leave on a scheduled flight. She sees Fifi, now injured and dying, and breaks down in tears. The Doctor is joined by Ace and they watch her cry. Their task is done. 

Priscilla P and Daisy K have been put to work repainting the city as the Doctor and Ace bid farewell to Susan Q and Earl Sigma. The TARDIS has been mostly repainted blue, and Ace finishes the job before they leave.

_{Synopsis from Doctor Who: The Seventh Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}