Josh Baskin, a 12-year-old New Jersey boy, finds that whenever he sees pretty 13-year-old Cynthia Benson, he is unaccountably speechless. He doesn't understand his new feelings, but every family on the street knows what has occurred. For Josh, childhood has ended; adolescence has occurred, and the long complex process of growing up has begun. ("Overture/Can't Wait"). Then Josh receives amazing news from his best friend, Billy Kopecki: Cynthia Benson thinks Josh is "cute." All Josh has to do is make a move tonight at the carnival ("Talk To Her/The Carnival."). But making his move does not turn out as planned. Meeting Cynthia in line for a ride called Wild Thunder, Josh musters enough courage to "talk to her", only to find that she has a date who is 16. Worse, Josh is not big enough to be allowed on the ride. Humiliated, Josh skateboards away-and finds himself in a secluded byway of the carnival with fun house mirrors and a mysterious arcade game, ZOLTAR SPEAKS. The mysterious figure in the arcade box instructs him to "Make a Wish!" Impulsively, Josh makes the only wish on his mind: "I wish I was big!" The machine produces a card: "Your wish is granted." A clap of thunder, sudden rain-Josh runs home.
The next morning, Josh wakes up-and sees in his mirror the face of a man in his thirties ("This Isn't Me"). Still a 12-year-old boy, he now inhabits the body of a grown-up. His mother thinks he is an intruder and drives him from the house. (In the touring and rentable versions, Josh's song is replaced by Mrs. Baskin singing about motherhood while making breakfast ("Say Good Morning To Mom").) Only Billy, his best friend, understands. Billy decides they must go to New York City, find an arcade with a Zoltar, and let Josh wish himself back to his old self. Arcades in New York, however, don't have any Zoltar Speaks, and locating carnivals will take six to eight weeks (three to four weeks in rented versions). Josh despairs at the prospect of remaining a grownup for that long, and worries that he will have to find a job. In the touring and rented versions, Billy tries to calm Josh, telling him that he'll be fine, because "You're A Big Boy Now". Billy returns to New Jersey, leaving Josh to spend his first night as a grown-up alone in the Port Authority Bus Terminal. (In the Original Broadway version, Josh then wishes he could go home ("I Want To Go Home").)
The next day, waiting for Billy under the clock of FAO Schwartz ("The Time Of Your Life") Josh meets MacMillan, the head of a toy company whose sales have suddenly plummeted. Josh, the 12-year-old that he is, tells MacMillan what his toys lack. What MacMillan sees, however, is a 30-something-year-old man with an amazing insight into toys, children and (when they discover a piano keyboard they can dance on) having fun ("Fun"). MacMillan offers Josh a great job. Josh enters the grownup world of business. His innocence causes chaos. MacMillan cancels the company's Christmas toy, which his employees say "can't miss". The company executives panic. Paul Seymour, V.P. in charge of product development, wants revenge for the cancellation of his toy("Josh's Welcome"). Susan Lawrence, V.P in charge of marketing, whose affair with Paul is just ending, finds herself attracted to Josh ("Here We Go Again"). (Note: In the touring and rental versions of the show, this scene opens with the executives singing about the toy business ("Welcome To MacMillan Toys"). Paul and Susan's songs have also been rewritten ("MacMillan Toys 2"), and Susan has an additional song regarding a wedding-obsessed secretary ("My Secretary's In Love").)
As a perk of his job, Josh is given a loft apartment. He furnishes it with toys. Susan arrives to make a pass at him (in the tour version, "Let's Not Move Too Fast"). Misreading her intentions, Josh innocently tries to find something they can do together ("Do You Want To Play Games"), and finally turns on a toy planetarium that fills the room with stars ("Stars, Stars, Stars"). Beguiled, Susan finds herself spending the night at Josh's-angelicly in separate bunks. In the tour version, Susan reminisces about her first love after Josh has fallen asleep ("Little Susan Lawrence"). At a company party, Paul learns of Susan's night with Josh and picks a fight with him. Susan comforts Josh and once again, his innocence wins her heart. MacMillian challenges his executives to come up with a new Christmas toy, and demands for them to find a way to relate to their children. Josh suggests dancing ("Cross The Line"). He gets everyone to do a line dance, during which Susan gives him a kiss unlike any he had ever received as a boy from his mother. When Billy arrives with the list of carnivals which has finally arrived, Josh chooses to go off with Susan. The adult world is beginning to attract him.
In a suburban mall, Billy, angry at being jilted by Josh, seeks the company of other kids ("It's Time"). He meets Josh's mother: today is Josh's birthday and the party would have been at the mall. Billy reassures her that Josh is coming home, which only makes Mrs. Baskin more aware of how fast Billy, and all the children have grown up ("Stop Time"). The mall turns surreal, and becomes the scene of the birthday Josh is missing ("The Night-Mare"). Josh wakes up in Susan's office. They have been up all night trying, unsuccessfully, to invent a toy. They try thinking as children instead of adults- Susan can't remember how she felt as a 13-year-old, but under Josh's prodding the memory returns ("Dancing All The Time"). The moment fills with emotion, Susan moves to Josh. The scene freezes. Young Josh appears to sing a duet with Big Josh about his inner feelings. ("I Want To Know").
The next morning, Josh barrels into the office as a new man ("Coffee Black"). His secretary, Miss Watson, is bowled over. Not only does Josh feel like an actual grown-up, but during the night, he has thought up a Christmas toy. The executives help him develop it for MacMillan. Billy returns with the list of carnivals, having located the Zoltar machine. Josh is, however, full of himself and does not want to return to his former size. Billy accuses Josh of betraying himself, but at that moment Susan appears and kisses Josh. Billy finally understands why he has been dismissed, and leaves. Susan invites Josh to a dinner party with her friends, and Josh excitedly exclaims that he is going to a real "grown-up dinner party with Susan's grown-up friends".
The party is a disaster ("The Real Thing"). Josh humiliates himself and realizes how far he is from being an adult. Susan takes this moment to tell Josh her true feelings for him ("One Special Man"), feelings that Josh is too young to return. (In the tour and rental versions, this is replaced by a reprise of "The Real Thing.") Josh tells Susan the truth: that he is really a 13-year-old boy. Susan, seeing only a grown man, assumes this is some kind of elaborate brush-off. It breaks her heart to have been wrong about another man. For the first time, Josh understands that being a grownup is more than just being big. It is being responsible - in this case, for someone who you care for and who cares for you in return. Josh realizes that everything you say and do carries a certain weight that a kid can't imagine when you're a grown up ("When You're Big"). He returns to his neighborhood, and watches boys and go through the first nervous motions of pairing-off events appropriate to being thirteen ("Skateboard Ballet"). Josh finds Billy and tells him he wants to go home.
Billy has found a Zoltar in a warehouse filled with discarded remnants of amusement parks. Josh asks Susan to meet him there- Before he can make his wish and leave the grown up world for the time being, he must say goodbye to make sure Susan understands. Arriving, Susan finally accepts the magic of what has happened to Josh. She tells Josh how he changed her life ("I Want To Go Home/Stars, Stars, Stars (reprise)") and, understanding that it's the only way, tells Josh to make his wish. As he does, Mrs. Baskin enters with Billy. Josh returns to being small, and as he and his mother embrace, the curtain closes with Billy examining the Zoltar machine. (In the touring and rental versions, the Finale number is "We're Gonna Be Fine")