Just 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 the game and a possible college basketball scholarship to propose to her.

20 years later, Mike (Matthew Perry)'s 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 (Thomas Lennon); he quits his job; and his high school-age children Maggie (Michelle Trachtenberg) and Alex (Sterling Knight) 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 in high school posing as “Mark Gold”, Ned's son, and plans to go to college with a basketball scholarship. As he befriends his children, however, Mike comes to believe that his mission is to help them. Mike comforts Maggie when her boyfriend Stan, the captain of the basketball team, dumps her. With his help, Alex overcomes Stan's bullying to obtain a place with Mike on the basketball team and the girlfriend he desires.

Through their children Mike spends time with Scarlett, who is attracted to his remarkable resemblance to her husband in high school. Mike has difficulty resisting his desire for her despite the relationship's apparent inappropriateness. At the same time, he resists Maggie's growing infatuation with him by telling her that he loves another. Meanwhile, Ned courts Jane Masterson, the high school principal; they discover a mutual interest in The Lord of the Rings.

Mike soon realizes that Scarlet is the “best decision” he had ever made and tries to re-unite with her and unsuccessfully explains to her that he is actually Mike, her husband.

The film ends with the re-union of Mike and Scarlet, on the day of the final match, re-making the scene of their proposal in 1989, Mike once again forsaking the chance at professional basketball in order to be with her. At the end of the film we see Ned and Jane in bed together with Mike walking in, revealing that he is now the High School's basketball coach.