Popular high school senior Mike O'Donnell (Zac Efron) seemingly has it all. He is a star athlete headed straight for a college scholarship when he decides to give it up to settle down with his high school girlfriend Scarlett.

Twenty years later, an adult Mike (Matthew Perry) finds his life is not exactly what he expected. He is separated from his wife, Scarlett (Leslie Mann), and living with his best friend Ned Freedman (Thomas Lennon), his career at a pharmaceutical company is at a stand still, and his relationships with his teenage children are nonexistent. After getting passed up for yet another promotion at work, he returns to his high school to reminisce over his basketball awards and the life he could have had. While he is reliving his past, he is approached by a janitor (Brian Doyle-Murray) and shares with him how things were so much better when he was 17.


As Mike is driving home from the high school, he sees the mysterious janitor standing on the ledge preparing to jump into the Los Angeles River. Mike rushes out of his car to rescue him, but when he gets there, the janitor has vanished. What Mike doesn't realize is that he is about to fall into the river and turn into his 17 year old self.

Young Mike (Zac Efron) returns to Ned's house, where he has the difficult task of convincing Ned that he is in fact Mike. At a loss of what to do, Ned pretends to be Mike's father and they register Mike back in high school to finish the life he never had the chance to live.


High school presents a lot of new challenges for Mike, such as dressing cool, keeping up with the latest gadgets, and making new friends. But nothing compares to being in high school with his own children. He discovers that his daughter, Maggie (Michelle Trachtenberg), is not nearly as innocent as he thought she was, she's dating the star athlete who picks on his son, Alex (Sterling Knight).


But Mike's most difficult realization is that he hasn't been a very good father or husband. Mike befriends Alex and tries to instill him with enough confidence to join the basketball team. He gets close to Maggie's obnoxious boyfriend and does everything in his power to break them up. Most importantly, he visits an unsuspecting Scarlett and rediscovers all the things that initially made him fall in love with her.

Through his experiences, Mike realizes that he had chosen the perfect life he just never appreciated it. Now, he must figure out a way to transform back into his older self and win back his wife and kids!