The novel opens with Chap, an uncooperative orphan, detained in the storeroom of a hostel for fighting with another boy. Two social workers arrive and, because of Chap's striking resemblance to a missing boy named Cassiel Roadnight (despite a couple of minor differences), assume he's Cassiel and attempt to reconnect him with his family. Chap, who is in fact not Cassiel, initially protests. However, he comes to realize that impersonating the missing boy will allow him something of a normal life, much better than his current homeless and friendless existence, so he accepts their help and takes on the role of Cassiel.
In a flashback chapter, Chap reminisces about his old life with his "Grandad," in which the two of them lived alone in a large house. Grandad was only ever reading and drinking inside, although he allowed Chap lots of freedom to do other things. It's revealed at the end of the chapter that the government program took him away because the situation was deemed unsuitable for a child.
Back in the present, Cassiel's sister, Edie, calls the office and talks to Chap, believing he's Cassiel. Chap does nothing to prove otherwise, and Edie drives down to pick him up. It appears that Chap bears enough of a resemblance to her brother that she believes his authenticity, and they have a slightly strained reunion (for her because of his reticence, for him because of his anxiety about being revealed).
In another flashback, the illustration of Grandad becomes clearer; he was often drunk, although Chap didn't know the difference, and he ventured out of his house only when Chap convinced him to go out with him.
Edie signs the paperwork and drives Chap back to her house. On the way, Cassiel's mother calls, and all involved parties have a slightly uncomfortable conversation. Chap adequately guesses and fakes his way through the awkward conversation with Edie until they arrive at the house, which is not Cassiel's old house because the family has moved since Cassiel's disappearance. At the new house, Chap "reunites" with Cassiel's mother, Helen, who doesn't figure out his deception either. Frank, Cassiel's older brother, calls while the family is at the house and talks to Chap for the first time. Chap figures out that Helen is an addict, and he moves into his room, which Edie tried to reconstruct exactly from their old house.
Chap had been a vegetarian, but in order to impersonate the meatball-loving Cassiel, he forced himself to eat meat. At dinner, Edie and Helen muse about how different Cassiel seems, but not different enough to arouse any suspicion on their part that he isn't who he says he is. Edie likes the change, saying that he seems nicer than when he disappeared. That night, Chap wonders about the tidiness and bareness of Cassiel's room before Helen comes in and gives him several photo albums out of nostalgia. Chap stays up late memorizing the images so he can better impersonate Cassiel.
The next day, Edie takes Cassiel to town (Hay-on-Wye, Valentine's home town) so people can see that he's back. The general reaction is complacent surprise, and Edie is disappointed that there isn't a bigger fuss. The word gets out, though. When they arrive back at the house, Frank is there, and he and Chap "reunite" as well. A couple of reporters for the newspaper arrive and gather pictures and information about Cassiel's return.
Over the next few weeks, Chap adjusts to life as Cassiel, but is generally on edge and anxious about being found out. He keeps to himself for the most part, refusing to go see Cassiel's old friends. Eventually he decides he can't do it any more and leaves the house, intending not to return. On the way out, though, he runs into Floyd, an eccentric guy his own age who is struck with surprise and fear at Cassiel's reappearance. He gives Chap some information about the night Cassiel disappeared and promises to meet him later at the clock tower.
Back at the house, Chap mentions Floyd, and the reaction of his family is immediate and stony. Edie makes Chap call him and cancel the meeting to appease Helen, but Floyd understands Chap's situation and says he'll be at the meeting anyway. Cassiel's family reveals that Floyd accused Frank of murdering Cassiel, and that's why they all hate him.
Chap meets with Floyd anyway, and in doing so accidentally blows by all Cassiel's old friends in favor of Floyd, earning angry remarks and confusion. Floyd gives him information that suggests Cassiel was in real danger the night of the fire, and Frank might have been seriously involved. He asks Chap to ask Frank about someone named Mr. Artemis. Edie shows up and berates Chap for meeting with Floyd considering his relationship to their family, feeling betrayed.
In a flashback, it's revealed that Grandad slipped and fell while he was out, and he never returned to the house. Chap lived by himself for three weeks before he was taken away.
Back home, Chap asks Frank about Mr. Artemis. Frank has a strange and terrifying reaction to the words, but keeps his mask up. He leaves, giving some excuse about a work emergency.
In his mind, Chap revisits his last memory of Grandad. He finds him in an assisted living institution, where he has dramatically physically and mentally deteriorated. Grandad reveals to Chap that they aren't actually related; he was a shy, socially anxious man in his 50s when a girl showed up at his door with a baby, saying she burned down a house. She left the baby (named Damiel) and departed a while later, and he took it upon himself to raise the little "chap." Chap/Damiel reacts angrily, confused about his identity, and leaves the old man.
Back in the present, Chap meets with Floyd again, who reveals that both he and Frank know that Chap isn't Cassiel. He tells Chap that Frank has been engaged in illegal activity, including murder and robbery, and that he killed Cassiel because he found out the source of his wealth and started blackmailing him. The two of them plan to take Frank down at the upcoming Hay on Fire festival, the same one at which Cassiel was killed the last year.
At home, Helen tells Chap that Cassiel was a twin; the other, supposedly dead in a fire, was named Damiel. Chap figures out that he is really Damiel and actually belongs in the family.
At the Hay on Fire festival, Damiel lets Frank know that he won't continue with the charade, and Frank responds with a death threat. The plan was to confront Frank and have Floyd record the whole thing on his camera. The confrontation occurs: Frank attacks Damiel, who retaliates and knocks Frank unconscious. Floyd was too late to record the video, but it turns out that Edie was hiding in the bushes, recoding the whole thing.
Damiel goes home, eventually taking his place in the wake of the news of Cassiel's murder and Frank's arrest.
