The last of the Yellow Kangs, hiding from the taunting jeers of the other girls in Paradise Towers, is found by something much worse...

The Doctor and Mel are on their way to Paradise Towers; Mel to visit the award-winning swimming pool (the one in the TARDIS was jettisoned after it began to leak), and the Doctor to see the Towers themselves, which won several architectural awards. But when they materialize in a courtyard in the Towers they're surprised to find it run-down and filthy, covered in graffiti and filled with rubbish. They are then captured by a street gang of young girls, the Red Kangs, who overheard them discussing the Great Pool in the Sky and are suspicious of their intentions. The Red Kangs' leaders, Bin Liner and Fire Escape, seem confused by the clash between Mel's red hair and blue clothing and aren't sure what colour Kang she's supposed to be. The Doctor is more concerned when they mention that the last of the Yellow Kangs has been "made unalive", and realizes that there's something else about Paradise Towers which the Kangs aren't telling him.

Young caretaker 345 stroke 12 subsection 3 is on patrol in the Towers when he finds a bloodstained yellow scarf, the last remains of the Yellow Kang. Worried, he tries to report in to the Chief Caretaker, but the Chief seemingly ignores his fears and orders him to continue on patrol. The young caretaker does so, but is trapped and killed by one of the robotic Cleaners as the Chief listens, satisfied that things are going well; more food is on its way to his little pet...

The Chief then sends his deputy and a squad of Caretakers to arrest the Red Kangs gathering in Fountain of Happiness Square, and in the confusion the Doctor and Mel are separated; the Doctor is captured by the Caretakers, and Mel, fleeing with the Kangs, finds herself lost in the corridors of the Towers. She happens to run into a pair of pleasant old ladies, who clear some suspiciously large bones down their waste disposal chute before inviting Mel into their apartment for tea and cakes -- just the thing to fatten her up. As they share tea and cakes, Mel asks Tilda and Tabby what happened to the Towers, but they're not really interested in digging up the past; all they remember was that the children and old people were sent to the Towers while the in-betweens went off to fight a war and never came back. Now life in the Towers has become desperate and people have to do whatever they can to survive... At that moment, a muscular young man, Pex, bursts into the apartment, looking for people to save from danger. Mel takes the opportunity to bid goodbye to the disappointed Tilda and Tabby, and Pex convinces her to let him accompany her as a guide. As they go, they witness a Blue Kang funeral for the last of the Yellow Kangs and Mel realizes that there is something terrible happening in the Towers...

As the Doctor tries to protest that he's not the Great Architect at all, the Chief selects a 327 Appendix 3 subsection 9 Death for him. But before sentence can be carried out, a report comes in about Caretaker 345 stroke 12 subsection 3's disappearance, and the Chief is forced to go out and investigate. He leaves his Deputy in charge of guarding the Doctor, and the Deputy does so by the book, ignoring the Doctor's attempts to inquire what's made the Cleaners go rogue, and why they should wish to kill the Great Architect upon his return. Eventually the Doctor convinces the Deputy to let him have a look at the rulebook, and, skimming through it, quotes an obscure rule which states that after Caretakers have been guarding a condemned prisoner for thirty-five minutes they must stand up, take five steps forward, close their eyes and put their hands on their heads. By the time the Deputy works out what's wrong with this scenario, the Doctor has stolen the key to the door and escaped.

Mel eventually realizes that Pex doesn't know his way around the Towers either and is just interested in showing off his strength and bragging about his bravery. Eventually she demands an explanation for his presence, and he reluctantly tells her that he was placed in the Towers by powers he's not allowed to name in order to serve and protect the populace. As they proceed, however, they are captured by a group of Blue Kangs, who saw Mel talking to the Red Kangs earlier and want to know what she has to do with them. Eventually they decide to let her go, but first tell her the truth about Pex; he's actually a coward who stowed away on the ship to Paradise Towers because he didn't want to fight in the war. Mel, disgusted with Pex for lying to her, storms off into the Towers on her own as the Kangs laugh at Pex and taunt him cruelly.

The Chief Caretaker makes a speech to the other Caretakers, telling them not to spread rumours about rogue Cleaners; but when he learns that the Deputy let the Doctor escape, he's furious and orders him to arrange a Towers-wide manhunt. He tries to calm himself by going into the Basement to speak to his pet, but is surprised to find a scrap from a Red Kang's outfit on the floor. Has someone else been feeding his pet without his permission? The glowing neon eyes on the tank in the centre of the Basement pulse angrily as the pet simply demands more food...

The Doctor finds more graffiti, this time a picture of a door marked "NO ENTRY" with smoke coming out from behind it. He finds a telephone on the wall and accidentally breaks it, causing coins to spill out -- and as he studies them he recognizes the name of the Great Architect, Kroagnon, from somewhere. Before he can work out where he's heard it before he is attacked by two Cleaners and falls through a trapdoor into the Red Kangs' "Brainquarters", where Bin Liner and Fire Escape hold him prisoner again. He's furious when they won't answer his questions about the "NO ENTRY" door, but they're in turn surprised when he reveals that the Caretakers are disappearing as well. He eventually wins them over to his side by showing them how to use the coins in a drinks dispenser to provide themselves with the "Fizzade" soft drinks. At that moment the Caretakers track him down and start burning their way into the Red Kangs' Brainquarters, and the Doctor must convince the Kangs that he didn't lead them there.

The Doctor offers to hold off the Caretakers while the Kangs escape, since it's his fault that they're here in the first place. The Deputy isn't happy when he breaks into the Brainquarters to find only the Doctor there, but he soon recovers and takes the Doctor back to the Chief's office. On the way he hears rumours that more Caretakers have disappeared and worries that the Cleaners really may be getting out of control, but the Chief refuses to discuss it, and sends him and the other Caretakers away while he and the Doctor have a Regulation 13 Appendix 2 Final Conversation.

As Tilda and Tabby prepare the cooking pot, Mel sees something moving about within the waste disposal unit, and when Tabby goes to investigate a metal claw emerges and drags her in. Tilda blames Mel and is about to stab her when Pex bursts in, looking for someone to save from danger. He's appalled when he realizes he's found exactly that, and stands shaking helplessly while Tilda throws her knife at him and heads back into the kitchen area to find more. He and Mel are saved when the claw emerges from the waste disposal unit again and pulls Tilda in, and Pex realizes to his surprise that he's just saved someone's life for the first time ever. Mel searches the apartment and finds a map of Paradise Towers, including the Basement, to which entry is prohibited on pain of death. The pool is on the uppermost floor of the building, and Mel doesn't listen to Pex's warnings that the Kangs say only the unalive go there. Wanting to prove himself, he agrees to help her reach the pool safely.

The Chief Caretaker admits that he now knows the Doctor isn't really the Great Architect. Nobody's ever seen the Great Architect, as he mysteriously disappeared after building the Towers. But the Chief has always feared the Architect's return; as it stands now he's the most powerful force of authority in the Towers, and he doesn't want to see things change. The Doctor realizes that the Chief has been allowing the Cleaners to kill people -- and that something else has begun doing this as well and the Caretaker doesn't know who or what. At that moment the Deputy breaks in with news of Tilda and Tabby's death, and the shaken Chief heads off to investigate in person. He leaves the Doctor with the Illustrated Prospectus, a video guidebook to Paradise Towers, but as the Doctor starts to watch it the Red Kangs break into the Caretakers' office, tie up the Deputy and his guards and rescue the Doctor.

Mel and Pex are chased into a lift by a Cleaner, and find the lift going out of control; the Kangs sometimes press all the buttons at once, and they could end up going up and down for hours before getting to the Pool. The lift breaks down, and Pex's attempt to fix it sends it plummeting to the Basement, where he and Mel hear the growling of the Chief's pet -- "Soon, soon, soon I shall be free!" The lift starts working again and takes them to the Pool at last. Mel decides to relax and swim, ignoring Pex's terrified warnings that the pool is dangerous. But for once, he's right -- a mechanical crab lies in wait at the bottom of the pool...

Back at the Red Kangs' Brainquarters, the Doctor watches the Illustrated Prospectus, which lists some more of the Great Architect's achievements. He recognizes the name of one, Miracle City, and finally remembers where he's heard Kroagnon's name before and what's going on. After building Miracle City, Kroagnon tried to stop anybody from moving in, claiming that they would ruin the beauty of his achievement; and when people did eventually move in they all ended up dead. Nothing was ever proven, and since he was a brilliant architect he found work elsewhere. Perhaps the people who commissioned Paradise Towers tried to prevent the same thing from happening here by trapping Kroagnon inside his own creation... and perhaps he's searching for a way out. Bin Liner and Fire Escape admit that Kangs have broken into the Basement and seen terrible things happening, and the Doctor decides to see for himself; but at that moment the Blue Kangs break into the Brainquarters, winning the Kangs' game. The Doctor manages to calm down the angry Kangs and convinces the Blue Kang leader to help him, Bin Liner and Fire Escape to investigate.

The Kangs rescue the Doctor, and they flee as the Chief marches stiffly out of the forbidden room in the Basement. Except he's no longer the Chief, as the Doctor well knows; he's now possessed by the mind of Kroagnon, and Kroagnon intends to send the Cleaners out to kill everyone in Paradise Towers and repair the damage the filthy human parasites have caused to his creation.

Mel enjoys her swim very briefly before being attacked by the crab. Pex is unable to work up the courage to shoot the crab, but he manages to pass his gun to Mel, who destroys the creature herself. The Doctor arrives with the Kangs, who are surprised by Mel's bravery and taunt Pex mercilessly when they realize he couldn't do anything to help. The Doctor explains the situation to Mel; the crab was just one of the traps set by Kroagnon, and if he'd had his way the Towers would have been filled with them. Now he's free to kill everyone in the complex, and as long as the factions continue fighting amongst themselves he'll find it easy sailing.

Maddy unexpectedly arrives with a few of the Rezzies, to apologize for their earlier behaviour; they were desperate and frightened, but some were worse than others, and the worst have gone now. The Kangs are suspicious and wary, but the Doctor convinces them to work with the Rezzies -- and even with Pex, who's still desperate to prove himself. When the Deputy Chief Caretaker arrives, having fled the Caretakers' office when the dead body of the Chief walked in with Kroagnon's mind animating it, the Doctor realizes that they're on their way to organizing a decent fighting force.

The Kangs arm themselves with dynamite from the Caretakers' rodent-hunting reserves, while the Rezzies plan to distract the Cleaners and tie them up with their knitting. The Red Kang Brainquarters will be rigged up as a booby-trap for Kroagnon, and Pex unexpectedly volunteers for the most dangerous duty of all -- leading Kroagnon into the trap. The Doctor contacts Kroagnon and drives him into a rage by denigrating the Towers and Kroagnon's architectural skills, and Pex then contacts Kroagnon and offers to lead him to the Doctor in return for being allowed to leave the Towers with his life.

As soon as Kroagnon has left the Caretakers' office, the Caretakers, Rezzies and Kangs begin fighting back against the Cleaners. Pex must lead Kroagnon into the trap as slowly as possible, to give the Doctor a chance to set it up while the others whittle down the opposition; but when Kroagnon begins to become suspicious, Pex panics and leads him to the trap as quickly as possible. As a result the trap isn't ready when Kroagnon arrives, and when the Doctor tries to push Kroagnon in, Kroagnon overpowers him and begins beating him senseless. Pex panics and flees, but sees Mel staring at him accusingly, and, ashamed, he runs back, pulls the Doctor out of the way and leaps into the booby-trapped Brainquarters, dragging Kroagnon with him. Both are killed in the explosion.