In 1989, 17-year-old Mike O'Donnell (Zac Efron) learns from his girlfriend Scarlet (Allison Miller) that she is pregnant right as he begins his high school championship basketball game. In a split-second decision, Mike abandons the game to pursue Scarlet, thus forsaking his aspirations for a career in professional basketball. His coach, Murphy (Jim Gaffigan), had previously advised him that he was on the brink of securing a full college scholarship if he continued playing at his current level.
Fast forward to 2009, Mike, now a 37-year-old man (Matthew Perry), finds himself in a rut. Scarlet, portrayed by Leslie Mann, has separated from him, consistently reminding Mike that his lifelong regrets stem from not attending college. This tumultuous period leads him to move in with his wealthy best friend since high school, Ned Gold (Thomas Lennon). Ned's success comes from creating software that revolutionized P2P music sharing and subsequently developed an opposing program to prevent it.
Scarlet points out that Mike can trace every failure in his life back to his decision to prioritize Scarlet over college. As the divorce looms in just two weeks, Mike feels marginalized at his job in a pharmaceutical company where he is significantly older than his co-workers. Despite excelling in sales for the past 16 years, his hard work is overlooked in favor of a younger employee. After a physical altercation involving his boss’s Bluetooth earpiece, he is fired, and his teenage children, Maggie (Michelle Trachtenberg) and Alex (Sterling Knight), seem to want little to do with him.
While taking a nostalgic trip back to his high school, Mike encounters a mysterious janitor (Brian Doyle-Murray), whose intervention magically transforms him back into his 17-year-old self. Upon returning to Ned's house and looking in the mirror, Mike realizes the magical transformation is real. Initially skeptical, Ned comes around once he sees a photograph of young Mike that corroborates their story.
Resolved to seize the opportunity of reliving his youth, Mike enrolls in high school again, adopting the identity of Mark Gold, Ned’s son, with the goal of securing a basketball scholarship for college. Meanwhile, Ned becomes infatuated with the principal, Jane (Melora Hardin), who regards him as nothing more than a nerd.
As he tries to connect with his children, Mike realizes that his newfound purpose is to guide them. He discovers that Alex is being bullied by Stan, who is dating Maggie, while Maggie plans to move in with Stan after graduation. In a series of events, Mike supports Alex in his aspirations and hoists him up to secure a position on the basketball team. Ned, persistent in his courtship of Jane, employs outlandish strategies to win her affection, yet to no avail.
Throughout this journey, Mike's relationship with Scarlet blossoms again, as she finds him eerily reminiscent of her high school sweetheart. He struggles with his own feelings, especially as Maggie misinterprets his teenage persona's advances. A pivotal moment occurs during a party at Ned's house where Mike confronts Stan for mistreating Maggie. After an altercation, he wakes up to find that Maggie is attempting to seduce him, leading to an awkward conversation where Mike expresses his feelings for someone else.
Scarlet’s arrival at the party highlights the complexities of Mike’s feelings, culminating in an attempt to kiss her which she rebuffs, leaving Mike to mend the pieces of his shattered marriage. On the day the divorce is about to be finalized, Mike seizes the chance to win Scarlet back by reading a fabricated letter from himself. This prompts her to pause the divorce proceedings, igniting a flicker of hope.
In a climactic basketball game, Mike reveals his true identity to Scarlet, leading to a frantic chase as she attempts to escape. With a profound revelation, Mike hands off the basketball to Alex right before he's transformed back into his 37-year-old self, leading to an emotional reunion with Scarlet.
As Mike prepares for a fresh chapter as the new basketball coach at his kids’ school, Ned revels in his newfound relationship with Jane, cementing a positive outlook for both their futures.
