In 17 Again, Mike O'Donnell (played by Zac Efron as a teenager and Matthew Perry as an adult) is a former high school basketball star who gave up his dreams of playing in college to marry his pregnant girlfriend, Scarlet (Leslie Mann). Now, almost 20 years later, Mike is stuck in a dead-end job, separated from Scarlet, and estranged from his teenage children, Maggie (Michelle Trachtenberg) and Alex (Sterling Knight).
After a chance encounter with an eccentric janitor at his old high school, Mike magically transforms back into his 17-year-old self. Unable to convince anyone of his true identity, he enrolls in his kids' high school and tries to fit in while reliving his glory days on the basketball court.
During his journey, Mike discovers that his son is being bullied and his daughter is dating the same manipulative jerk he once was. He also realizes that his marriage fell apart because he took Scarlet for granted and failed to appreciate her. In trying to fix these problems, Mike not only discovers the importance of family and friendship but also finds a new direction in life.
In the end, Mike learns that it's never too late to make things right and that even small changes can have a big impact on the future. He also reunites with Scarlet and learns to appreciate the life he has, rather than longing for a second chance.