The film begins with Karen Powell (Diane Lane) going into labor during a Halloween party and being rushed to the hospital by her husband, Brian (Brian Kerwin), and their friends. Although the delivery is successful, the baby is premature, born after only ten weeks of pregnancy, and is diagnosed with an exaggerated form of Werner syndrome as stated by Dr. Benfante (Allan Rich) and Dr. Lin (Keone Young). According to them, as this very rare autosomal recessive disorder progresses, Jack will age at a rate four times faster than normal children due to his internal clock that seems to be developing faster.
Ten years later, Jack (Robin Williams) is next seen as a 10-year-old boy in the body of a 40-year-old man, with a group of four boys telling possible stories of a "monstrosity" of a boy their age that can't go to school. He scares them away by dipping a fake eye into slime and throwing it at them from his window. He is extremely childish as a consequence of his secluded life. He has only had contact with his parents and tutor, Lawrence Woodruff (Bill Cosby), who introduces the idea that he should go to public school. His parents initially balk at the idea of him doing so because he could be emotionally hurt.
When he first attends school, he is exploited by the other kids to win at basketball against bullies, and eventually to get adult magazines and other such items. As time goes by, he is accepted by them, beginning with Louis, as they discover that they like him. He attempts to be normal there, for example, when he deals with his first crush, heartbreak, and the relationship with his teacher, Miss Marquez (Jennifer Lopez). When he goes through a fall while attempting to leave, he is rushed to the hospital, where his doctor explains that he suffered a shocking severe strain (which could've been a rare form of angina), and also, because of his Werner syndrome, his internal clock is starting to run out. Realizing the dangers it might entail for his health, his parents decide to withdraw him from school, which upsets him.
He sneaks out of the house and goes to a bar, where he gets drunk and befriends a man named Paulie (Michael McKean), and tries to hit on Dolores (Fran Drescher), Louis's mother. However, he then gets into a fight with a bully (Edward Lynch) and are both arrested. Dolores bails Jack out, leaving the other guy to suffer in jail. Upon returning home, he locks himself in his room and doesn't go out for weeks. Karen speculates that perhaps he realized the fragility of his life and is now scared of facing the outside world again. He also doubts the need to study as he realizes that he wouldn't have the time to use any of the knowledge.
Meanwhile, his friends continue coming to his house, hoping that he will come out and play, but he refuses. Finally, Louis has an idea: he brings the entire class over as they take turns yelling "Can Jack come out and play?" and participate in various games and fun activities right in front of the yard. The next day he decides to go back to school.
Seven years later, an elderly-looking Jack and his four best friends are at their high school graduation. He delivers the valedictory speech, in which he reminds his classmates that life is short, and urges them to "make your life spectacular," as the four of them drive off into the future.Robin Williams as Jack PowellDiane Lane as Karen PowellBrian Kerwin as Brian PowellJennifer Lopez as Miss MarquezBill Cosby as Lawrence WoodruffFran Drescher as Dolores DuranteAdam Zolotin as Louie DuranteTodd Bosley as EddieSeth Smith as John-JohnMario Yedidia as GeorgeMichael McKean as PaulieAllan Rich as Dr. BenfanteKeone Young as Dr. LinAllison Whitbeck as LucyDwight Hicks as High School PrincipalMark Coppola as Radio Personality (voice)Al Nalbandian as Principal McGee
Jack debuted at #1 and grossed roughly $58.6 million on a budget of $45 million.
Jack received negative reviews. Rotten Tomatoes, a review aggregator, reports that 17% of 30 surveyed critics gave the film a positive review; the average rating is 4/10. Todd McCarthy of Variety called it a "tedious, uneventful fantasy" that wastes the talents of the filmmakers.
The film theme is "Star", performed by Canadian rocker Bryan Adams.