A young boy named Josh Baskin, feeling overlooked and belittled because of his age, makes a wish to be "big" at a mysterious carnival machine named Zoltar. The next morning, he wakes up to find himself in the body of an adult. Shocked and unprepared, Josh, now in a man’s body, struggles to maintain his childlike identity while navigating the complexities of adulthood. Without the ability to prove his true age or identity, he flees his hometown, leaving behind his family and his best friend, Billy.

In the city, Josh lands a job at a toy company due to his genuine childlike insight into what makes toys fun. His innocence and enthusiasm quickly earn him a series of promotions, captivating the company's top executives, and he becomes a valuable asset to the team. Meanwhile, he begins to enjoy the perks of adulthood, from his own apartment to a budding romance. However, as he immerses himself deeper into his new life, the challenges and responsibilities of being an adult start to weigh on him, leading Josh to reflect on the true meaning of happiness and the essence of his wish.