The Doctor and Melanie, looking for a swimming pool, land in Paradise Towers, a luxurious 22nd century high rise apartment building now fallen into disrepair and chaos. The building is divided between roaming gangs of young girls called Kangs, grouped in colour theme, and the Doctor and Mel encounter the Red Kangs. They have just discovered the death of the last Yellow Kang and are plotting how to attack the Blue Kangs. Elsewhere in the Towers, one of the Caretakers - who act as 'Judge Dredd' style policemen – is hunted down and killed by a robotic cleaner, which appeals to the sadistic Chief Caretaker when he overhears the death.
The Chief sends a squad of Caretakers to arrest the Red Kangs and in the ensuing confusion the Doctor is split from Mel and captured by the Caretakers. Mel meanwhile heads off to one of the still occupied apartments in which two elderly ladies ('rezzies') live. Tilda and Tabby explain that all the able bodied men left the Towers to fight a war, leaving behind only the children and the elderly. The only other man still loose in the Towers is Pex, a would-be hero, who appoints himself Mel's guardian.
At the Caretaker control centre, the Doctor meets the Chief Caretaker, who greets him as the Great Architect, designer of Paradise Towers, and then promptly calls for him to be killed. The Doctor cites an imaginary rule from the Caretakers manual, confusing them enough to make his escape. Mel and Pex meanwhile have headed to the top of the building, and are captured by a party of Blue Kangs. Before the pair are freed the Kangs reveal to Mel that Pex survived by fleeing from the war.
The Doctor finds the Great Architect is named Kroagnon, and is reunited with the Red Kangs. They explain that Kangs and Caretakers have been disappearing in ever greater numbers. While the Doctor is being interrogated, the Caretakers track him down to the Red Kang headquarters and attempt to break down the door to their headquarters. Elsewhere Mel has visited Tilda and Tabby again and soon finds herself under threat when it emerges they are cannibals and plan to eat her.
The Doctor succeeds in holding off the Caretakers long enough for the Kangs to flee. Meanwhile Tabby and Tilda are delayed in their eating of Mel when they are disturbed by a noise in the waste disposal. It turns out to be a metal claw, which first drags Tabby to her death in the disposal system, and then Tilda. Pex arrives and somehow succeeds in saving Mel. Mel and Pex find a map of the Towers and decide to venture to the roof, where the luxury swimming pool is located.