In 1998, Evan Treborn (Ashton Kutcher), who endured horrific sexual abuse at the hands of his girlfriend Kayleigh’s (played by Amy Smart) father, alongside traumatic experiences such as witnessing his mentally ill father's death, struggles with frequent blackouts during moments of high stress. As a child (portrayed by Logan Lerman) and a teenager (by John Patrick Amedori), Evan's chaotic past leaves him with gaps in his memory. His concerned mother, desperate to understand his condition, seeks medical help, but the tests yield no answers. A doctor recommends that Evan maintain a journal, hoping to illuminate the reasons behind his blackouts.
Despite his efforts, Evan often leaves situations just before crises occur. A vivid example of this is when he, together with Kayleigh, her brother Tommy, and their friend Matt, attempts a prank involving explosives stolen from Kayleigh’s father, which leads to a catastrophic injury for Matt. Desperate to remedy his chaotic recollections, Evan's mother tries hypnosis, yet it only results in troublesome nosebleeds each time he attempts to revisit the painful memories.
The tension escalates when Tommy discovers Kayleigh sharing a kiss with Evan and retaliates by inflicting a brutal and unspeakable act against Evan's dog, which Evan is unable to remember as he passes out at the critical moment. After this incident, Evan's mother decides to move them to a different town.
While attempting to entertain a girl in his college dorm room, Evan realizes a shocking truth: reading from his journals enables him to travel back in time, allowing him to modify distressing parts of his past and unintentionally affecting his present life. He recalls that during the explosive mailbox incident, a young mother and her baby were killed when they opened the mailbox just as the explosive detonated, causing serious injury to Matt as he heroically tried to save them.
His motivations to manipulate his past are rooted in his desire to save Kayleigh from being abused by her father and tormented by her sociopathic brother, Tommy. In one of his initial time-traveling attempts, he confronts Kayleigh's father in the past and attempts to make him aware of his repugnant actions. When Evan awakens in an alternate reality, he discovers that he is married to Kayleigh, but the pleasure is short-lived as memories of a troubled future threaten to overwhelm him.
In this alternate reality, while Kayleigh escapes her father’s clutches, her brother Tommy has unleashed his wrath with devastating consequences. When Tommy eventually confronts Evan, the ensuing violence leads Evan to jail. There, he is besieged by Tommy’s former associates, pushing Evan to once again utilize his journal for time travel, this time to prevent his dog’s previous torment.
As he navigates through these alternate realities, Evan's decisions yield tragic outcomes: Tommy's demise at Matt's hands and Kayleigh sustaining severe facial injuries. Every adjustment he makes to remedy past tragedies leads to increasingly dire consequences in his present life. Ultimately, realizing that each encounter brings about suffering not just for himself but for those he loves, Evan begins to understand the true depth of his actions.
In his final stand to re-do the past, Evan journeys back to when he first met Kayleigh. He resolves to keep her out of his life altogether, believing this to be the ultimate solution to protect her. In an emotionally charged conclusion, he destroys all his journals, eliminating the temptation to meddle with their lives once again. The film closes eight years later, with Evan walking through Manhattan, where he unexpectedly crosses paths with Kayleigh. They lock eyes for a brief moment, leaving Evan with the poignant choice to let her go without a word, finally grasping the weight of his decisions.
