The protagonist is 12-year old Matthew Amsterdam. His 16-year old brother Greg, is making a documentary about how lame Matt is and his 15-year old sister Pam, joins in with play-by-play. Even the family dog, a dachshund named Biggie, hates the kid. Matt for some reason is scared of the small wiener dog.
Matt tries to reason with his mother to let him move into the guest room, which is twice as big as his small room. She tells him that the guest room is for guests. While he grasped the concept without her explanation, he still thinks that their only annual guests, his grandparents, wouldn't mind sleeping in his room. Then over dinner, Greg continues his documentary on how much Matt sucks and when Matt gets huffy, Matt's the one who gets punished. That night, Greg and Pam sneak the dog into Matt's room and it bites him on the face.
The next day, Matt decides that since his single mom works late at a second job, she'll never know if he sneaks off to sleep in the guest room. Matt's plan to fall asleep works perfectly. However, his plan to wake up the same as he fell asleep runs into a hitch, as he wakes up as a 16-year old.
Much to his surprise, Greg and Pam are now 12 and 11 and just as annoying. Shocked to discover no one remembers how life used to be, Matt finds himself stuck in a new life. His mother drops him off at the high school, where he immediately gets threatened by a bully. In English class, there's some "comedy" at the expense of Anna Karenina.
Matt has a lot of trouble adjusting to his new body. He keeps running into walls and tripping over his feet. He also knocks out a girl with a volleyball during gym class. In the hall between classes, Matt runs into the bully again. Matt realizes that high school can be a scary place. He decides to leave before he encounters more typical high school situations, such as peer pressure or knocking up Manny. On his way out of the school, he bumps into a cute twelve-year-old girl with a ponytail named Lacie. He knocks her down a few times by accident.
That night, Matt must again sleep in the guest room. When he awakes, he's pleased to discover he's a twelve-year-old again. He's less pleased to discover his parents have been replaced with complete strangers and he's now an only child. He gets dropped off at a different middle school and runs into Lacie again. Because of overpopulation, the school had to add more lunch periods and so Lacie's is at 8:30AM.
Lacie and Matt decide to eat outside and they're enjoying their brunch when two boys in leather jackets take a break from leaning up against cars to chase after Matt. Lacie holds the street toughs off while Matt makes his escape. Back at home, he tries to call his relatives but they don't exist, so there's no one to accept the charges. He's also a total jerk to his new mom for no reason. He tells her to mind her beeswax.
Matt goes to sleep in the guest room and wakes up to discover he's eight. And he has a pet monkey. And he wears a blue spandex suit. And lives with an extended circus family. His irate lion tamer father insists Matt practice the new lion riding trick, and so he tries to throw his son into a cage with a lion. Matt makes a break for it nd hides underneath a truck in the parking lot. Then he runs into the two leather-clad toughs and they chase him back to the same lion cage. He runs inside and hides behind the lion. He threatens to sic the lion on the toughs if they come any closer. When they don't believe him, he does in fact sic the lion on them.
That night, Matt gets very excited about falling asleep, thinking that maybe he'll wake up as a sports superstar or in a different book. No such luck. Matt wakes up and discovers he's an old man. He rushes back to sleep to will another fate for himself. This new reality is only marginally better, as he wakes up to find he's now a seven-foot lizard monster. Monster Matt has sharp teeth and horns and striped oozing lizard skin. He flees his house and starts accidentally terrorizing his neighbors, causing car crashes and the townspeople begin to swarm away from this monster. Feeling only marginally more ostracized from others than he was at the beginning of the book, Matt adjusts remarkably well to being a lizard monster. He stops a speeding car with his claws and begins to eat it piece by piece. He's munching on a car door when he spots Lacie, who leads him away from the onlookers. They run down alleys and backways until they come across an isolated house. Lacie leads Monster Matt into the house and into the hands of the two leather-clad street toughs, who thank her for her work. Then they throw a magical net over the lizard monster.
The three lead the netted monster into a jail cell inside the house. When Matt wakes up, he's a fourteen-year-old boy. Finally Matt and the reader are given some answers regarding what's happening. See, when Matt slept in the guest room, he accidentally triggered a reality warp. This is revealed to Matt as though it were obvious. Lacie proceeds to explain that by triggering a reality warp, every time Matt wakes up, he changes reality for everyone in the universe. In the liminal justice system, reality-based offenses are considered especially heinous. Lacie and the two toughs-- who are named, Bruce and Wayne-- are members of an elite squad known as the Reality Police.
The Reality Police decide that the only way to stop Matt from changing reality is to put him to sleep forever. He thwarts their plan however by falling asleep and waking up as a squirrel. He escapes through the bars of the jail cell window and flees into the night. He decides that if he can just make his way back to his home and fall asleep in his old room again, he can undo all the events of the book.
An extended sequence follows between Matt the Squirrel and his sister Pam. Pam tries to keep the squirrel as a pet, which works fine for Matt because he thinks he can just squirrel into his room, go to sleep, and wake up cured. However, this plan fails and Matt the Squirrel barely escapes being locked inside a hamster cage. He climbs up a tree in the front yard and falls asleep. When he wakes up, the tree limb he was resting on as a squirrel crashes down, due to Matt now being a morbidly obese child.
Fat Matt tries to gain entry to his house by ringing the doorbell and asking if he could sleep in their house. This plan doesn't work. So the fat kid runs outside, climbs up the tree and attempts to jump onto his bedroom ledge from two stories up. Thrilling action commences as the fat kid jumps and then dangles from the gutter by his fingertips, managing to land on the ledge before he could fall to his death. He successfully breaks into his house and falls asleep in his bed.
Matt wakes up and he's back in his old room. Everything is just as it was. Matt realized in their absence that he does love his family, even though they can treat him lousy at times. Matt is so caught up in celebrating his safe return to reality that he forgets that it's his birthday. When he arrives home from school, his mother surprises Matt by revealing that she's moved all his stuff into the guest room, which is now his room. Matt responds by screaming like a little girl.