Evan Treborn (JOHN PATRICK AMEDORI) is a young and troubled boy. Raised by his mother, Andrea (MELORA WALTERS), due to his father being institutionalized, Evan has various bouts of blackouts where he can't remember anything. A doctor suggests that he start keeping diary-like journals, but they don't do much good in recovering some deeply repressed memories.
Most of them involve his best friends, siblings Kayleigh (IRENE GOROVAIA) and Tommy Miller (JESSE JAMES) as well as Lenny Kagan (KEVIN SCHMIDT). With a prank gone terribly bad and some unsavory moments involving Mr. Miller (ERIC STOLTZ), a video camera, and child porn, it's no wonder Evan isn't quite right.
He isn't alone. Years later, while Evan (ASHTON KUTCHER) is a college student with a Goth roommate, Thumper (ETHAN SUPLEE), Lenny (ELDEN HENSON) still appears shell-shocked from a past event; Kayleigh (AMY SMART) has been out on her own since the age of 15 and brother Tommy Miller (WILLIAM LEE SCOTT) is a jerk.
As Evan tries to recall the past with aid from his childhood journals, he ends up imagining a certain event in a slightly different fashion. To his shock and amazement, that minor change mysteriously alters the present. Armed with such knowledge and apparent power, he sets out to return to the past - in his mind - and right the various wrongs that occurred there. Yet, as he does so, he also creates various unexpected ramifications that radically change the history and personality of his friends and those around him.
As things progressively begin to spiral out of control, Evan races to return to the past in hopes of fixing those changes he accidentally created, only to have more of them occur.