At a carnival with his parents and baby sister, thirteen-year-old Josh Baskin begs to ride a roller coaster alone. In line for the ride, he sheepishly strikes up conversation with a pretty schoolmate, Cynthia Benson, but is crushed when her older boyfriend shows up and is further embarrassed when a carnival worker tells him he is too short for the ride. Roaming the carnival grounds by himself, Josh chances upon an automated fortune teller and inserts change into the machine. The eyes of Zoltar, the fortune teller, become red, and the machine instructs Josh to make a wish. Josh wishes to be big, and receives a printed card that reads, “Your wish is granted.” The next morning, Josh wakes up to find he has become a full-grown man overnight. Panicked, Josh rides his bicycle to the carnival grounds but finds it empty. Returning home, he scares his mother. Believing that the adult version of Josh is an intruder, Mrs. Baskin threatens to call the police. Josh goes to his school and corners Billy, his next-door neighbor and best friend, after a gym practice. Once Billy is convinced of Josh’s story, he steals cash and some of his father’s clothes for Josh, and accompanies him on a bus ride to New York City. Meanwhile, Josh’s parents file a missing child report. In New York, Billy helps Josh find a room at a cheap hotel and promises to come back the next day. That night, Josh remains awake in his squalid room, frightened by the noises coming from the other rooms and the street below. The next day, Billy and Josh search the city for another Zoltar machine, to no avail. At a consumer affairs office, they apply for a list of carnivals and fairs, but the clerk informs them that the request will take six weeks to process. Resigned to staying in the city for the next six weeks, Josh answers a job notice for a computer operator at Macmillan Toys. Although he lies about his social security number and work history, Josh is hired. Posing as a friendly kidnapper, he calls his mother and promises that her son will be returned in the same condition he was taken. She demands proof that Josh is all right, asking what song she used to sing to him. He responds correctly by singing the show tune, “Memories,” causing Mrs. Baskin to weep. On a weekend, Josh runs into Macmillan, the head of Macmillan Toys, while playing around in FAO Schwartz, a toy store. Recognizing Josh as a new employee, Macmillan reveals that he comes to FAO Schwartz every Saturday. Macmillan then asks Josh’s opinion on various toys. They happen upon a giant set of piano keys on the floor, and Macmillan watches while Josh jumps around on the keys. Josh encourages Macmillan to join him in a performance of “Chopsticks,” and a crowd of onlookers applauds them. When Macmillan promotes Josh to Vice President of Product Development, two of his coworkers, Susan and Paul, stew over the unfair promotion. Soon after, Paul presents a new toy at a meeting, and Josh questions the product’s appeal. Afterward, Paul tells Susan, who is also his girl friend, that Josh is a “killer.” Josh rents a large, loft apartment and transforms it into a playroom with arcade games, a trampoline, and a basketball hoop. He writes his parents an upbeat letter, stating that his time away has been similar to summer camp. At a Macmillan company party, Josh arrives in a flamboyant white tuxedo, and his colleagues snicker at him. Susan befriends him and encourages Josh to try caviar, but he chokes on it in disgust. She suggests they leave the party, and they take Susan’s hired limousine to Josh’s apartment. Although Susan tries to have a serious adult conversation, Josh is distracted by the amenities in the limousine and encourages her to stick her head out of the sunroof. When Susan mentions spending the night together, Josh misinterprets her sexual advances and welcomes her to sleep over. Susan is overwhelmed by the childlike décor of Josh’s apartment. Despite her reluctance, Josh lures Susan onto the trampoline, and they have fun jumping together. Later, Susan is disappointed when Josh indicates that they must sleep on separate bunks of his bunk bed. Upset that Susan left the party with Josh, Paul takes Josh to play racquetball the next day with the intention of humiliating him on the court. However, Josh loses patience with Paul’s poor sportsmanship and they get into a tussle. Back at the office, Susan nurses Josh’s cuts. Josh tells her she is one of the nicest people he has ever met and she kisses him on the cheek. Soon after, Susan breaks up with Paul. Billy takes Josh to a restaurant for his birthday, but feels neglected when Josh admits he has other plans later that night. Josh goes on a date with Susan to an amusement park where he does not notice another Zoltar machine. At a dance hall, Susan confesses that Josh has been on her mind and embraces him on the dance floor. Josh begins to confess his real age, but stops short, kissing her instead. Back at her apartment, Susan undresses while Josh watches in awe and they spend the night together. Billy finally receives the list of carnivals and fairs that he and Josh requested, but he cannot reach Josh at the office. At Susan’s apartment for dinner, Josh talks excitedly about a toy idea, but she interrupts to ask about the state of their relationship. Instead of answering her question, Josh playfully wrestles Susan to the ground. The next day, Billy bursts into Josh’s office, but Josh claims he is too busy and asks his friend to come back later, prompting Billy to accuse him of losing sight of his priorities. Later, Josh returns to his hometown and watches his friends and neighbors from afar. At dinner with Susan, Josh reveals his real age and attempts to explain his transformation, but she refuses to believe him. Having researched the list of carnivals and fairs, Billy returns to Josh’s office and informs him there is a Zoltar machine at Sea Point Park, the park he visited with Susan. Although Josh and Susan are due to make a presentation about a computerized comic book they developed, Josh wanders out of the presentation in a daze and Susan rushes after him. On the street, Josh slips into a taxi, and Susan arrives moments later, noticing Billy as he calls after Josh from the sidewalk. Susan confronts Billy, demanding to know where Josh is headed. Josh finds the Zoltar machine and makes a wish to be a kid again. Susan arrives and reprimands him for walking out on her. She sees the Zoltar machine and realizes that Josh was telling the truth. Softening, Susan offers to drive him home, and outside his house, she kisses his forehead before they part ways. As Josh heads to his front door, Susan sees him transform back into a thirteen-year-old boy. Josh reunites with his mother, and sometime later, he and Billy discuss baseball as they walk down the street.