Seventeen-year-old Mike O'Donnell has been big success in high school。 He seemingly has it all, when, right before the championship basketball game, his girlfriend Scarlet Porter finds out that she is pregnant。
In that moment, he makes the decision to throw everything away (including basketball and a chance at a scholarship) and proposes to her。
Twenty years later, Mike's life has come to a standstill。 Scarlet has separated from him, forcing him to move in with his geeky and wealthy best friend since high school, Ned Gold。
His job is going nowhere, and his kids Maggie and Alex want nothing to do with him。 While visiting Hayden High School to reminisce about the life he threw away, he encounters a mysterious janitor。
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 is a thief。
Ned fights with Mike, as Mike tries to prove he is the real Mike。 When Ned tries to hit Mike over the head with a picture of the two of them at their 1989 graduation, he realizes that is 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, 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。
With "Mark's" help during the basketball game, Alex makes the winning basket。 Meanwhile, Ned is attracted to the principal of Hayden High, Jane Masterson。
Although his initial attempts to 'peacock' her fail he manages to take her out to dinner。 When "Mark" notices the two of them leaving he decides to throw a party at "Ned's" house to celebrate Alex scoring the winning basket and to help him get the girl he likes。
During the party Scarlet appears, looking for Alex, and "Mark" tells her that Alex could have a girlfriend by the end of the party and brings her up to the balcony to show her。
Ned and Jane soon bond over a love of The Lord Of The Rings。 When he takes her back to his house, they discover an out-of-control party raging there。
After Maggie tries to have sex with "Mark", Maggie sees their principal Ms。 Masterson, who threatens everyone with detention, unless the stop the party, and goes looking for "Mark" to warn him and finds Mark with her mother Scarlet。
While "Mark" and Scarlet are talking, Mike's feelings for her are reawakened, (especially after she tells him "he's really become a part of the family")。
But when he kisses her, she is appalled, and slaps him and calls him a "weirdo little man child" when he chases after her and tries to tell her that he is Mike。
This exchange is witnessed by Maggie and her friends, who are all disgusted and slap him too, especially since Maggie has become smitten with "Mark"。
For cutting his date with Jane short, Ned slaps "Mark" a few times as well。
The following morning, Ned reminds Mike that it is the date of his divorce hearing with Scarlet。
"Mark" appears in court to read a letter from Mike O'Donnell pleading for a second chance。 Scarlet is touched by the letter。
When she sees that the "letter" is really just a piece of paper with directions on it, she realizes that "Mark" spoke extemporaneously。
Later, at the championship game, Mike makes a gesture which she recognizes as his。 Realizing the truth about who "Mark" is, and that she is in the same situation as twenty years earlier, she flees the scene。
Mike follows, handing the ball to Alex, who goes on to make the winning shot。 The janitor catches sight of Mike and changes him back into his adult self。
Mike promises Scarlet to spend the rest of his life making it up to her。 Scarlet and Mike kiss and reunite。
In the end, Mike happily reconciles with his family, and becomes the coach of the Hayden High basketball team, while Ned and Ms。
Masterson seem to have a "relationship"。