Plot
In 1989, 17-year-old star basketball player Mike O'Donnell finds out that his girlfriend, Scarlet Porter, is pregnant. She tells him about her pregnancy moments before his likely scholarship-clinching high-school championship basketball game. Mike plays the first few seconds of the game before walking off the court to go after Scarlet, abandoning his hopes of going to college and eventually having a professional basketball career.
Twenty years later, 37-year-old Mike finds his life stagnant and boring and Scarlet has filed for divorce. He moves in with his life-long best friend Ned Gold, who is wealthy, geeky, and also unhappy.
After being passed over for a promotion at work he believed he deserved, Mike expresses his frustration to his boss, which ends with him being fired. His high-school-age children, 18-year-old Maggie and 16-year-old Alex, want nothing to do with him.
Mike revisits the high school's memorabilia case, finding his senior year team photo. A janitor comes by, and he vents about his life, saying he wished he had a do over. Later that night, while driving in the rain, Mike comes across the janitor on a bridge who falls in. He leans over to look for him, falls in, transforming him back into his 17-year-old self.
After becoming convinced of Mike's identity, Ned believes his transformation was caused by a mystical spirit guide who is trying to steer him on a better path. Mike enrolls in high school by posing as Mark Gold, Ned's son. He plans to go to college on a basketball scholarship.
As Mike befriends his own son Alex, he discovers that he is being bullied. Mike also learns that his daughter Maggie has a boyfriend named Stan, who does not respect her and frequently torments Alex. Mike comes to believe that his mission is to help his children.
Through their children, Mike spends time with Scarlet, who notes his remarkable resemblance to Mike in high school, but rationalizes it as an odd coincidence. Attempting to fix his relationship with Scarlet, Mike begins to finish all the garden projects he had abandoned as an adult. He does his best to separate Stan and Maggie while encouraging Alex to be more confident. Mike has difficulty resisting his desire for Scarlet despite the relationship's clear inappropriateness.
Mike throws a party to celebrate a basketball game win at Ned's house. He confronts Stan, who has recently dumped Maggie for refusing to have sex with him. Mike gets knocked out and wakes up to Maggie trying to seduce him. He tells her he is in love with someone else, and Maggie leaves, much to Mike's relief.
Scarlet arrives at the party, and Mike shows her that Alex has finally managed to get together with his crush. The two have an intimate conversation where Mike, caught up in the moment, starts to kiss Scarlet. Disgusted, she storms off as he tries unsuccessfully to explain his true identity.
On the day of the court hearing to finalize the divorce, Mike makes one last attempt to win Scarlet back (as Mark) by reading a supposed letter from Mike. After he exits, Scarlet notices that the "letter" is actually directions to the courtroom; she begins to grow curious and postpones the divorce by a month.
Frustrated that he could not save his marriage, Mike decides to once again pursue a scholarship and move on with a new life. During a basketball game, Mike reveals his true identity to Scarlet. As she runs away, he decides to chase her down, just like he did in 1989 before transforming back into his 37-year-old self. Mike happily reunites and reconciles with Scarlet, realizing that she is the best decision he ever made.
As Mike prepares for his first day as the new basketball coach at his kids' school, Ned—who has successfully started a relationship with Jane, the principal—gifts him a whistle. Both are happy with the fresh starts in their lives.
