In the year 1998, Evan Treborn (Ashton Kutcher), who suffered severe sexual abuse (sexual abuse by the father of his girl friend Kayleigh from school) and traumas (death of his mentally ill father, in front of his eyes) as a boy (played by Logan Lerman) and a teenager (played by John Patrick Amedori), blacks out frequently, often at moments of high stress. His mother has him examined by a doctor, but all the tests fail to spot the issue. The doctor suggests that Evan keep a journal to help him with the blackouts. Evan has the habit of backing out just before the most unpleasant events in his life, including the time when his girlfriend Kayleigh, her brother Tommy & their friend Matt stole an explosive from Kayleigh's father stash & tried to explode as a prank in a neighbors mailbox, which somehow led to a serious injury to Matt.Evan's mom tries hypnosis, but Evan simply gets nosebleeds when forced to remember the blackout portions of his life. Later Tommy finds Kayleigh kissing Evan & takes it out on Evan by burning Evan's dog alive. Evan is blocked out on that event as well (right after the point when he tries to intervene to try & save his dog). Tommy beats up Evan badly & prompts Evan's mom to change towns.While entertaining a girl in his dorm room, he finds that when he reads from his adolescent journals, he travels back in time, and he is able to "redo" parts of his past, thereby causing the blackouts he experienced as a child. He also remembers that during the mailbox incident, the resident of the house (a mom with her baby) approached the mailbox while the explosive was still on trigger, & she opened the mailbox just as it exploded, killing them both. Matt tried to save them & was seriously injured.There are consequences to his choices, however, that then propagate back to his present life: his alternate futures vary from college student, to prisoner, to amputee. His efforts are driven by the desire to undo the most unpleasant events of his childhood which coincide with his mysterious blackouts, including saving his childhood sweetheart Kayleigh (Amy Smart) from being molested by her father (Eric Stoltz) and tormented by her sadistic brother Tommy (William Lee Scott).His first attempt to alter reality is when he goes back to the time when Kayleigh's father sexually abused him & her by making a soft porn movie of them. He "abuses" Kayleigh's father & gets him to realize the error of his ways. He wakes up in an alternate reality, where he is married to Kayleigh & instantly gets a huge headache where all the "new" memories of his alternate past come back to him. In this reality, Kayleigh was spared by her dad, but the dad took all of his sexual frustrations out on Tommy. Now Tommy is out of Juvi & is hunting to scare Evan away from Kayleigh. When Tommy attacks, Evan retaliates & ends up killing Tommy & goes to jail.In jail he is attacked by Tommy's Juvi buddies, & this time Evan uses the journal to travel back to the time when Tommy burnt Evan's dog alive. He convinces Matt to pick a piece of metal to cut open the ropes of the sack in which his dog is trapped. Evan talks Tommy into sparing his dog by saying that he would go to Juvi & he wouldn't want to leave Kayleigh alone with his dad. Tommy relents & releases the dog, when Matt kills Tommy with the metal shard. Kayleigh gets a gash on her face from the same metal, when she tries to save Tommy.Evan wakes in an alternate reality in his dorm room again. In this reality, Tommy is dead & Matt is institutionalized. Evan meets Matt in the hospital & Matt blames Evan for putting the metal shard in his hands, saying that EVan knew fully well that Matt would kill Tommy. Kayleigh is ugly due to a huge gash on her face & turns out to be a prostitute.Now Evan goes back to the time of the dynamite exploding & tries to warn off the woman and the baby approaching their mailbox. He gets close to the mailbox & ends up saving their life, but when he wakes up he has lost his arms & Kayleigh is having sex with Matt. Evan is distraught that in this reality he has even lost Kayleigh. He asks Kayleigh how she feels about him & she tells Evan that she hated her dad, but didn't opt to go with her mom (after their divorce) because she wanted to be with Evan. Thats when Evan realized that the only way to "fix" everything was to push Kayleih away in the first place.The actions he takes, and those he enables others to take during his blackouts, change the time-line in the new future where he awakes. As he continues to do this he realizes that even though his intentions are good his actions have unforeseen consequences. Moreover, the assimilation of dozens of years' worth of new memories from the alternate time-lines causes him brain damage and severe nosebleeds. Ultimately, he decides that his attempts to alter the past end up only harming those he cares about. But Evan realizes that the main cause of everyone's suffering in all the different time-lines is himself.At the conclusion of the film, Evan purposely travels back in time one final time to the first day he met Kayleigh. By scaring her away, he makes sure that she is never a part of his life, and thus finally succeeds in saving her life. He then destroys all of his journals and memories so that he is not ever tempted again to make any more changes. The film ends eight years in the future with Evan leaving an office building in Manhattan and passing Kayleigh on a crowded sidewalk. They alternately pause and turn after spotting and passing each other. After a moment of hesitation, Evan lets her walk away without speaking to her.