Evan Treborn (Ashton Kutcher), who suffered severe traumas as a boy (Logan Lerman) and a teenager (John Patrick Amedori), blacks out frequently, often at moments of high stress. While searching for an answer to heal his emotional wounds, he finds that when he reads from his adolescent journals, he travels back in time, and is able to essentially "redo" parts of his past, thereby causing the blackouts he experienced as a child. There are consequences to his choices, however, that he then propagates back to the present: his alternate futures vary from frat boy to prisoner to amputee. His efforts are driven by the desire to undo the most traumatic events of his childhood which coincide with his blackouts, including saving his childhood sweetheart Kayleigh (Amy Smart), from being abused by her father.
The actions he takes, and those he enables others to take during his blackouts change the timeline whereupon he wakes up in his new future. As he continues to do this, he realizes that even though his intentions are good, the actions he takes have unintended consequences. Moreover, the assimilation of dozens of years' worth of new memories from the various alternate timelines he has caused are causing brain damage. Ultimately he decides that his attempts to alter the past end up severely harming those he cares about. He travels back in time once more to the first day he met Kayleigh and warns her to stay away from him, going so far as to threaten her. He succeeds in undoing his childhood as he knew it. The film ends eight years in the future with a well-dressed Evan leaving an office building and passing Kayleigh on the street, Evan turns around while Kayleigh is still walking, she fails to notice him and carries on walking. Evan carries on walking to his planned destination upset, and Kayleigh turns around and sees Evan from behind, but does nothing.