Right before his 1989 high school-championship basketball game, 17-year-old Mike O'Donnell's (Zac Efron) girlfriend Scarlett (Allison Miller) tells him that she is pregnant. He leaves behind the game and a possible college basketball scholarship to propose to her.
Two decades later, Mike's (Matthew Perry) life has stalled. Scarlett (Leslie Mann) has separated from him due to his regrets about abandoning college, forcing him to move in with his geeky and wealthy best friend since high school, Ned Gold; he loses his job; and his kids Maggie and Alex want nothing to do with him. While visiting Hayden High School to reminisce, an encounter with a mysterious janitor (Brian Doyle Murray) transforms Mike into his 17 year-old self.
Given the chance to relive his life, Mike enrolls (under the name Mark Gold, posing as Ned's son) at Hayden High and comes to believe that his mission is to help his children. With Mike's help, Alex gets a place with Mike on the basketball team and the girlfriend he desires, and Mike comforts Maggie when her bully boyfriend dumps her. Through their children Mike spends time with Scarlet, who is attracted to his remarkable resemblance to her husband in high school. Meanwhile, Ned courts Jane Masterson, the high school principal; they discover a mutual interest in The Lord of the Rings.
At the championship game, Mike makes a gesture which Scarlet recognizes. Realizing who "Mark" is, she flees to prevent Mike from again leaving the game. Mike follows, as the janitor changes him back to an adult. Mike promises Scarlet that he will be a better husband and father. Reconciled with his family, Mike becomes the new school basketball coach.
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