In the city of Hokesh, a young street urchin named Joey Quine is having a familiar dream, one in which his dying body lies rotting on the plains, while alien creatures like jackals tear at his flesh. He awakens to find two other homeless teens, one human and one tentacled Dracori, are trying to rob him. When his life is threatened, he suddenly “remembers” a way in which he can escape. Old Man Srescht, the owner of the flophouse in which Joey was sleeping, responds to the subsequent screams to find that Joey’s attackers have been driven mad with terror, and when questioned by two agents of the City Patrol’s Black Watch, the mad boy claims to have been touched inside by the Broken Avatar.

Joey has fled out into the streets to consider what he’s done, but he avoids resting against the buildings; as a child he once saw a wall eat a person alive. As he navigates the streets, he realises that he can sense the flow of life throughout the city, and that he can control the thoughts of the passers-by. He vows not to abuse this power, which feels like something he’s remembering rather than something newly developed. As he tests his new abilities, acquiring money, food and clothing from the vendors and citizens of Hokesh, he attracts the attention of Sloater, the being who currently controls the life of the city. Sloater used to be human until his survey team stumbled across something interesting on this planet; now, he has lived for centuries, and he’s had a lot of experience in dealing with threats like Joey.

Joey needs to find somewhere to sleep, and therefore looks into the mind of a well-dressed man-about-town to learn how to go about it. What he sees inside the man Smith’s mind appalls him, and he realises that Smith must be stopped. He thus waits for Smith to make an assignation with a lady of the night, intending to follow them home; however, he’s caught off guard when Smith hails a hansom cab, something which Joey could never have done in his old life. He still hasn’t fully come to grips with his new circumstances, and he realises only now that he could have reported Smith to the City Patrol and used his powers to make them believe him.

By the time he tracks Smith down, it’s too late; Smith has already cut the woman’s throat, in a room festooned with the remains of his former victims. Having successfully lured Joey into his clutches, Smith’s eyes glow red, and a vast alien power pins Joey in place. Joey seems to see dream images of his childhood terrors, while trapped before a vast being of mutable flesh and clockwork. However, he also sees a woman who calls herself the Angel of Oblivion, who claims that she’s here to help him. This is an echo of the future, and something which Sloater wasn’t expecting, and it helps Joey to send a burst of energy back into the creature which is holding him in place. He breaks free of his nightmare to find himself standing in an ordinary house, a dead body before him and a bloody knife in his hand. Agents of the Black Watch break in and attempt to arrest him, but he senses an emptiness within their minds and realises that they are only puppets of another power. He overrides that power’s commands, and leaves the patrolmen tearing at each other while he flees to safety. Outside, however, he meets a woman who claims to be waiting for him; she is the Angel of Oblivion, and she introduces herself as Ace.

Elsewhen, a stranger in a mysterious blue box arrives in the Radiant City, a shining golden beacon of beauty. All inhabitants of the city worship their glorious leader, Magnus Solaris, who always has been and always will be. The stranger is here to destroy this utopia, but for the best of reasons; Magnus’ attention is wandering, and parts of the city are decaying into ruin. The ordinary citizens can’t perceive these rotting areas, and when the stranger begins to question their worship of Magnus Solaris, the concept that their leader may be imperfect is nearly enough to drive them mad. Magnus Solaris begins to sense disturbances as the stranger walks about his city, spreading doubt and uncertainty, and assuring the dying inhabitants of the rotting sectors that soon a Child will arise to put things right. Once started, the decay spreads quickly, and soon Magnus Solaris can no longer sense the life of the City as he once did. He must disguise himself in clothing that seems seedier and less fine than it ever did before, and go out into the City to see the truth of things for himself.

Ace explains to Joey that she was told to wait for him at a certain place and time, but instead she tried tracking him down earlier and ended up losing his trail. Now that she’s found him, she takes him to a house where he will be safe from the force hunting him. According to Ace’s friend, Joey’s power is simple precognition, but this close to the time-warping effects of the galactic core, it has the ability to change the world. Joey is bewildered by the alien manner of Ace’s speech, and by her disconcerting habit of swallowing food rather than simply chewing it and spitting it out as everybody else in the City does; nevertheless, over the next few days, he hides out in the house with her, testing the limits of his power, and slowly coming to realise that there are holes in his perception of the City -- as if the City is decaying. He also tries to read Ace’s mind, but can make no sense of the alien images within; however, he does see that her friend is occasionally named Smith, which is somewhat disturbing...

Eventually, Joey’s mind brushes up against Sloater’s, and Sloater immediately locates him and lashes out. Ace is just returning from a shopping expedition when every person on the street suddenly turns and heads for the safe house as if possessed; fortunately, she gets there first, helps Joey to escape, and detonates a half-kilo of explosives to destroy all alien technology in the house so the people of the City can’t get it. Joey, realising that their attackers were ordinary people who had been possessed, is upset with Ace for putting their lives at risk -- which, he realises, means that he’s showing real concern for other people for the first time in his life. The Power is changing the way he thinks. At Ace’s suggestion, Joey uses his Power to locate the one place in the sewer where Sloater can’t find them -- the sewers. Ace, who is starting to appreciate the pared-down simplicity of this City, realises that since the people here don’t actually eat, they don’t need sewers; the sewer is simply here because cities are supposed to have sewers.

The lifts in Magnus Solaris’ Gutter Palace are no longer working, and Magnus Solaris must climb down uncountable flights of stairs to reach the City. Outside, he sees that the dissolution is spreading, and when he steps into one of the decaying areas he sees an oddly familiar stranger speaking to the dying, mutating inhabitants, and telling them that soon they will live again through the Child. When the monstrosities recognise Magnus Solaris, they turn on him in rage, despite the stranger’s attempts to calm them. Magnus Solaris flees back to the palace, but he knows that it too will fall. The stranger is waiting there along with the blue box, and Magnus Solaris finally recognises him and remembers everything that he’d made himself forget. It’s time for him to give up his City; the Child will put it to rights, while Magnus Solaris allows the stranger to take him somewhere else.

Joey finds himself leading Ace through the sewers, following the tendrils of what appears to be a plant composed of animal matter. In the centre of the sewers, they find something like a mass of decaying human flesh, but Ace realises that it’s the exact opposite -- it’s as if this is where the people of the City are assembled, out of raw materials. In another sense, it’s as if the thing itself isn’t really there; this is just how Ace perceives something which is too alien for her to truly comprehend.

Joey communes with the thing, and learns or remembers more about his true identity; he is the Broken Avatar, imperfect but necessary. With this knowledge, he leads Ace to the Outmarsh, where those who have been forgotten by the City are allowed to exist; once every year, on the Feast of Fools, they march through the City and are recognised. Here, they find Ace’s friend, the Doctor, waiting with an impossibly handsome young man who flies into a rage upon seeing Ace and attacks her. Joey tries to defend her, but the Doctor pulls him and Magnus Solaris apart, telling them that violence only leads to one fighting oneself. He now reveals that the City is really a living organism, and the “people” within are symbiotic parasites. Normally the Cities on this world are only about a metre across, but when human colonists became a part of the evolutionary process, Hokesh grew to the size it is now. The City requires an Avatar to liaise between itself and its inhabitants, and the current Avatar, Sloater, is centuries old and is losing interest in life; thus, the City is dying with him. Sloater must be replaced, but he is struggling to hold onto the reins of power. Thus, Joey is not the new Avatar -- he’s just a stopgap measure, bred by the City to get rid of Sloater so the true Avatar can grow in peace.

Today is the Feast of Fools, which means that now Sloater can see the Outmarsh and locate his enemy. The Doctor, Ace, Joey and Magnus Solaris accompany the parade into the city, where they find Sloater and the Black Watch waiting for them. Sloater tries to direct the inhabitants of the City to tear Joey apart, but the Doctor produces a device which blocks his control of the inhabitants. Despite his weariness, however, Sloater has been the Avatar for too long simply to surrender, and he thus sucks the life force from all of the City’s inhabitants surrounding him, intending to blast Joey to ashes. Joey realises that he can do the same to Sloater, but he doesn’t know how. However, at the last moment, Magnus Solaris throws himself between Joey and Sloater, and as he takes the blast meant for Joey, Joey realises that he couldn’t read Solaris’ mind because it’s exactly the same as his -- in every sense. Solaris is blasted to ash, but in his last moment of life, his mind finally meshes with Joey’s, teaching Joey all he needs to know. Joey thus instinctively draws the life force out of everyone else in the City, and channels it into Sloater, destroying him.

Ace is horrified when she realises that everybody in the City is now dead, but the Doctor tells her that they were never really alive; they never really ate, because they weren’t real people, just parasites going through the motions of life on the surface of the true living City. Joey, however, is sickened by what he’s done, and he takes his leave of the Doctor and Ace, vowing to put things right. He will use his Power to shed his old, bad memories, remake himself as a new man, and rebuild the City in his own image. But it will only be temporary, until the true Avatar, the Child, arises to take his place...