In 1989, Mike O'Donnell (Zac Efron) was a big success in high school. He seemingly had it all, when, right before the championship basketball game, his girlfriend Scarlet Porter (Allison Miller) informed him she was pregnant. In that moment, he made the decision to throw everything away (including basketball and a chance at a scholarship) and proposed to her.


Twenty years later, Mike's (Matthew Perry) life has come to a standstill. Scarlet (Leslie Mann) has separated from him, forcing him to move in with his geeky and wealthy best friend Ned Gold (Thomas Lennon). His job is going nowhere, and his kids Maggie (Michelle Trachtenberg) and Alex (Sterling Knight) want nothing to do with him. While visiting Hayden High School to reminisce about the life he threw away, he encounters a mysterious janitor (Brian Doyle-Murray). On the way home, he sees the janitor standing on the edge of a bridge - apparently about to commit suicide, Mike runs towards him, but his path is blocked momentarily by a passing truck. When he reaches the edge, the janitor is gone, and there is a strange vortex below the bridge. Mike slips and falls into the vortex, blacks out, and awakens back on the bridge. When he gets home, he discovers that he has been magically transformed into his 17-year old self.


Ned sees Mike in his house, and thinks he's a thief. Ned fights with Mike, as Mike trys to prove he's the real Mike. When Ned trys to hit Mike over the head with a picture of the two of them at their 1989 graduation, he realizes that's the real Mike. Mike then returns to the school to find the janitor that he spoke with the day before and became aware of there being no such person working there. Mike then decides that with Ned posing as his father, Mike enrolls at Hayden High as "Mark Gold". He believes he has been given the chance to live his life over again, "but to do it right". Then he discovers that Maggie is dating the basketball captain Stan (Hunter Parrish), who is bullying Alex. He realizes that his real mission is to help his children, and makes friends with both Alex and Maggie. With "Mark"'s help, Alex gets a place on the basketball team and the girlfriend he desires. "Mark" comforts Maggie when she is dumped by Stan, who was pressuring her for sex. "Mark" also meets Scarlet. He helps her decorate her garden, gaining a new appreciation for her, and dances with her before she goes on a date (to the song that he and Scarlet danced to at their wedding). "Mark" and Scarlet clearly feel a connection as they danced, and are about to kiss, until Alex walks in on them. At first Scarlet seemed uncaring that her son is right there, but then she feels embarrassed and tells "Mark" that she is Alex's mother and that this is inappropriate.