A los 35 años, la vida de Mike (Mathew Perry) no ha ido por el camino del éxito que todos preveían cuando en el instituto era la estrella del equipo de baloncesto y el que ligaba con las chicas más guapas. Después de 18 años vendiendo productos farmacéuticos no ha conseguido ascender en el trabajo, no tiene buena relación con sus dos hijos Maggie (Michelle Trachtenberg) y Alex (Sterling Knight), y se acaba de separar de Scarlett (Leslie Mann), su encantadora mujer. Cómo le ha podido ir todo tan mal?
Una noche de tormenta Mike ve al que fuera bedel de su instituto subido encima de la valla de un puente mirando al río. Detiene su coche con intención de evitar que el hombre se tire pero cuando llega a la altura del bedel, éste ha desaparecido y Mike cae irremediablemente al río. Cuando a la mañana siguiente Mike llega a casa de su amigo Ned (Thomas Lennon), se da cuenta de que no es el Mike vendedor de medicamentos si no Mike O'Donnell, el fornido adolescente de 17 años (Zac Efron)...

At 17 Mike O'Donell was on top of the world; he was the star of his high school basketball team, and was a shoo in for a college scholarship. And is dating his soul mate, Scarlett. But on what's suppose to be his big game wherein college scouts are checking him out, Scarlett reveals that she's pregnant. Mike decides to leave the game and asks Scarlett to marry him which she does. During their marriage, Mike could only whine about the life he lost because he married her. So she throws him out. And when he loses his job, he returns to the only place he's happy at - his old high school. And while looking at his high school photo, a janitor asks him if he wishes he could be 17 again and he says yes. One night while driving he sees the janitor on a bridge and apparently jumps in and he goes after him. When he returns to his friend, Ned's house, where he has been staying, he sees that he is 17 again. He decides to take this opportunity to get the life he lost.