Twelve-year-old Matthew Amsterdam is sick of his life. He constantly gets bullied, especially by his high school-aged brother, Greg, and his older sister, Pam. Even the pet dachshund, Biggie, hates Matt. Matt asks his mother to let him move into the guest room, which is twice as big as his small room. She refuses, telling him that the guest room is for guests. The next night, while his mother is at work, Matt sleeps in the guest room for the night, foolishly believing that he won't be caught. Matt wakes up the next day and finds that he's 16 years 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 his new life and must endure a day at high school.
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, has trouble reading Anna Karenia in his literature class, and twice gets bullied by an unnamed boy. On his way out of the school, Matt bumps into a cute twelve-year-old girl with a ponytail named Lacie. That night, Matt sleeps in the guest room again. When he awakes, he discovers he's twelve years old again, but Matt is now an only child with a mother and father (it was established early in the book that Matt's father was dead). He gets dropped off at a different middle school and runs into Lacie again.
Lacie and Matt enjoy their brunch (the new middle school Matt is in is so overcrowded that children often have lunch in the morning) outside -- until two boys in leather jackets chase after Matt. Lacie holds the bullies off while Matt makes his escape. Back at home, Matt tries to call his real family, but is shocked to learn that they don't exist in any capacity. Again, Matt goes to sleep in the guest room and wakes up to discover he's an eight-year-old living with an extended circus family. His irate lion tamer father insists Matt practice the new lion riding trick, and tries to throw his son into a cage with a lion. Matt makes a break for it and hides underneath a truck in the parking lot. There, he runs into the two leather-clad toughs again, and they chase him back to the same lion cage. He runs inside and hides behind the lion. He threatens to sick the lion on the toughs if they come any closer. When they don't believe him, he does.
That night, Matt gets very excited about falling asleep, thinking that maybe his next reality will be better. Matt wakes up and discovers he's an old man. He rushes back to sleep to will another fate for himself. When Matt wakes up, he finds that his new reality is only marginally better, as he's now a seven-foot lizard monster with sharp teeth, horns, and striped oozing lizard skin. He flees his house and starts accidentally terrorizing his neighbors, causing car crashes. The townspeople begin fleeing from Matt and the ensuing disaster he's causing. Feeling only marginally more ostracized from others than he was at the beginning of the story, 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. As he munches on the car, Lacie appears and leads him away from the onlookers. The two 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. The toughs throw a magical net over the lizard monster. Lacie and the three leather-clad punks lead the netted monster into a jail cell inside the house where Matt falls asleep.
Matt wakes up and finds himself in the body of a fourteen-year-old boy. Despite being relieved over being human again, Matt demands Lacie to tell him what's happening to him. Lacie explains that she and the two leather-clad boys -- named Bruce and Wayne -- are members of The Reality Police, an elite squad of people hired to bring down anyone who alters the fabric of reality. In this case, Matt altered reality when he spent the night in the guest room of his house, which was actually a portal into a multitude of other realities, and the more Matt slept, the more skewed reality became -- not just for him, but for everyone else in the world. Now that Matt has been caught for committing this crime, the Reality Police have only one choice to keep him from perpetrating this crime: put Matt in a permanent sleep.
Matt 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 actual bedroom again, he can undo everything that's happened. Squirrel Matt doesn't get very far, as his sister, Pam, captures him and keeps him as a pet in her room. Matt escapes from the hamster cage Pam keeps him in and scurries out the window. He falls asleep on a tree branch and wakes up in the body of a morbidly obese boy. After falling out of the tree in the Amsterdam yard, Obese Matt tries to get in the house the traditional way: by ringing the doorbell and asking his mother if he can come in so he can sleep in his bedroom. Since Matt's mother doesn't recognize her son in this reality, Matt gets the door shut on him. Matt returns to the tree in the yard and climbs it, hoping that, despite his girth, he can reach his bedroom window and squeeze himself inside. Matt successfully gets in (after a few near misses) and falls asleep in his bed.
Matt wakes up and find that everything is back to normal. He's in his own body and is his own age. His brother and sister are at their normal ages, and his mother is single and widowed. Matt is overjoyed to have his family back, despite their poor treatment of him at times. After school, Matt's mother reminds him that it's his birthday and that she finally got him what he's always wanted: the guest room as his room.
- The two officers of the Reality Police are named Bruce and Wayne. This is a reference to the Batman comics, where Batman's secret identity is Bruce Wayne, a billionaire.
- Instead of Matt wanting to move into a guest room, it's an attic.
- Lacie, Bruce, and Wayne as the Reality Police are replaced by a black man and a white man as Men in Black-style secret agents.
- In the book, the Reality Police capture Matt and force him to drink a potion that will put him in eternal sleep. On the TV version, Matt is placed under arrest for warping reality, goes to trial for not being happy with his reality, and sentenced to be tossed in a portal.
- In the book, Matt is sent to a new reality every time he goes to sleep. On the TV version, Matt is sent to a new reality whenever the Reality Police go after him.
- Matt's realities in the book were: a high schooler, an only child with a different family, the son of a circus performer, an old man, a Godzilla-like reptile, a fat kid, and a squirrel. On the TV version, it was a hockey player, a brain surgeon, a bomb defuser, and a groom to an ugly bride.
- In the TV version, a German flag and Brazilian flag can be seen flanking the judge. It is never explained why the flags are there or why those two particular countries were chosen.
- In the book version, Matt freaks out when his mom surprises him with the guest room as his new room. On the TV version, Matt decides not to sleep in the attic and goes back to get his things. While there, he mutters that his reality is boring, and the Reality Police officers come back to take him away to court.