Harold Crick is an Internal Revenue Service (IRS) agent living a solitary life of strictly scheduled routine. On the day he is assigned to audit an intentionally tax-delinquent baker named Ana Pascal, Harold begins hearing the voice of a woman narrating his life. When his wristwatch stops working and he resets it using the time from a bystander, the voice narrates that this action will eventually result in Harold's death.
Harold consults a psychiatrist, who suggests he see a literary expert if he believes there is a narrator. He visits literature professor Jules Hilbert, who initially dismisses him. However, he recognizes omniscient narrative devices in what Harold claims the voice said, and is intrigued. He tries to help Harold identify the author, and determine if his story is a comedy or tragedy.
As Harold audits Ana, he develops an attraction to her, but when he obliviously rejects a gift of cookies because it could be considered a bribe, he takes it as a sign that he is in a tragedy. Jules tells Harold to spend the day at home doing nothing, and his living room is destroyed by a demolition crew that went to the wrong building. Jules takes such an improbable occurrence as proof that Harold is no longer in control of his own life, and advises he enjoy the time he has left, accepting whatever destiny the narrator has for him.
Harold takes time off from work, takes guitar lessons, moves in with his co-worker Dave, and starts dating Ana. Because she loves him, he reevaluates his story as a comedy. While meeting with Jules, Harold sees a television interview with author Karen Eiffel, and recognizes her voice as his narrator's. Jules, an admirer of Karen's work, says that all of her books are tragedies: the protagonist always dies. Karen has been struggling with writer's block on her next book because she cannot figure out how to kill Harold Crick, but has had a breakthrough and has begun writing again.
Harold telephones Karen and stuns her by accurately recounting her book to her. They meet in person, and she explains she has outlined the conclusion, but has not yet typed it in full. Her assistant, Penny, recommends that Harold read the outline, but he cannot bring himself to do so, and gives it to Jules. Jules deems it Karen's masterpiece to which Harold's death is integral, and he consoles Harold that death is inevitable, but this death will hold a deeper meaning.
Harold reads the outline and returns it to Karen, telling her the death she has written for him is beautiful and he accepts it. He takes care of some errands, and spends his last night with Ana. The next morning, Harold goes about his routine again, as Karen writes and narrates.
Karen reveals that when Harold reset his wristwatch, the bystander's time was three minutes fast, so he reached the bus stop early. A boy riding a bicycle falls in front of the bus; Harold runs into the street to save him, and is hit himself. However, Karen, traumatized by the idea that she unwittingly narrated real people to their deaths, cannot bring herself to finish the sentence declaring him dead. She meets Jules and offers him a revised ending.
Harold, heavily injured, wakes up in a hospital, and learns that shrapnel from his wristwatch — which was destroyed in the collision — blocked his ulnar artery and saved him from bleeding to death. Jules thinks this new ending weakens the book. Karen replies that the book was about a man who did not know he was going to die, but if Harold knew and accepted his fate, he is the kind of person who deserves to live. Karen's narration closes the film over a montage of Harold's newly invigorated life, ending on the ruined wristwatch that saved his life.
