In Penny Marshall's charming comedy, a 13-year-old boy named Josh (David Moscow) wants, more than anything else, to be "big." And when he makes a wish in a carnival booth, his dreams come true--he becomes a 30-year-old man (Tom Hanks), though his mind and spirit remain that of a child. Since he can't really go to school looking like an adult, and his mother doesn't know him in his new guise, Josh heads with his pal Billy (Jared Rushton) to New York, where they proceed to goof off, play around, and act like the kids they are. But when Billy leaves, Josh is subjected to the encroaching needs and responsibilities of adulthood, and he quickly discovers both the pleasures and the problems of being grown-up.
Josh makes a wish on a carnival wishing machine and gets more than he bargained for when he wakes up next morning to find that his wish to be 'big' has been granted. After landing a job as a toy company executive, his adventures start when he attracts a fellow worker with a jealous boyfriend. All Josh wants to do is play with the toys!