In the year 1998, Evan Treborn (Ashton Kutcher), who suffered severe sexual abuse and traumas as a boy (played by Logan Lerman) and a teenager (played by John Patrick Amedori), blacks out frequently, often at moments of high stress. 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. 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 (William Lee Scott).
The actions he takes, and those he enables others to take during his blackouts, change the timeline 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 timelines 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 timelines 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.