Robin Williams stars as Jack, a boy who ages at four times the rate of a normal child. Aged 10, he has the body of a 40-year-old man, and when sent to school is shunned by his class-mates as a freak. They gradually come to realise the advantages of having a friend who looks like a grown-up, however, and Jack soon finds himself winning basketball matches and posing as the Principal to fool one of his friends' mother.