The story unfolds with Karen Powell (played by Diane Lane) going into labor unexpectedly during a lively Halloween party. Rushed to the hospital by her husband Brian (Brian Kerwin) and their friends, the delivery goes well, but the baby is born extremely premature, just ten weeks into the pregnancy. Astonishingly, once examined, the infant is discovered to appear as a healthy, full-term baby. After various tests, the baby, named Jack, is diagnosed with Werner syndrome—a rare autosomal recessive disorder—with doctors Dr. Benfante (Allan Rich) and Dr. Lin (Keone Young) informing the parents that Jack will age at an accelerated rate, four times faster than an average child. Dr. Lin aptly describes his condition as “sick as frig.”
Fast forward ten years, Jack (played by Robin Williams) is seen as a ten-year-old boy trapped in the body of a 40-year-old man. As he peers out his window, he catches sight of four boys outside who are whispering about the “monstrosity” living there. Playing harmless pranks, such as tossing a fake eye into slime at them, Jack displays his playful spirit, though he remains extremely isolated, having only interacted with his parents and his tutor, Lawrence Woodruff (Bill Cosby). Lawrence brings up the notion of Jack attending public school, which terrifies his parents due to their concerns over potential emotional harm for Jack.
His first day at school is rocky; most kids cast him aside, intimidated by his adult appearance. With some encouragement from his dad, they agree to install a basketball hoop, which helps him gradually blend in. One day, Louis, a kind-hearted boy, picks Jack to be part of his basketball team, leading them to victory against some bullies. Soon after, Louis invites Jack into their circle, introducing him to their adventures, which includes fetching adult magazines—a strange yet amusing undertaking for a boy like Jack, struggling with his identity.
In an attempt to live like a typical child, Jack experiences his first crush on his teacher, Miss Marquez (Jennifer Lopez), but heartbreak ensues, leading him to fall down a flight of stairs, resulting in severe injury. At the hospital, the doctor reveals concerns about Jack’s health and his accelerated aging. Distressed by this revelation, his parents decide to pull him from school, much to Jack's dismay.
Feeling trapped, Jack sneaks out to a bar and, in a moment of recklessness, drinks too much, befriending a man named Paulie (Michael McKean), before engaging in a confrontation with a bully, which results in their arrest. His mother, Dolores (Fran Drescher), ultimately bails him out, comforting him as they return home. However, Jack retreats to his room, shutting the world out for weeks. Karen speculates he has come to understand the frailty of his life and now fears facing the outside once more.
His friends, undeterred by his absence, come over, hoping to coax him out. In a touching gesture, Louis brings the entire class to his house, encouraging them to chant “Can Jack come out and play?” and partake in outdoor games right before his yard, ultimately persuading Jack to return to school.
Seven years later, a now elderly-looking Jack stands with his best friends on graduation day. In a heartfelt valedictory speech, he reflects on the fleeting nature of life, urging his classmates to “make your life spectacular.” The scene crescendos with the five friends driving off into their promising future.
