Mort Rainey, a successful author, retreats to a secluded cabin in the woods of upstate New York to recover from a painful divorce from his wife, Amy. Struggling with writer's block and a deepening depression, Mort's isolation is interrupted by the arrival of a menacing stranger, John Shooter, who accuses him of plagiarism. Shooter leaves Mort with a manuscript that bears an uncanny resemblance to one of Mort's own stories, "Secret Window," but with an ending that Mort had never written. The confrontation sets off a disturbing series of events as Mort endeavors to prove his innocence and maintain his sanity.

As Mort digs deeper to find evidence supporting his original authorship of the contested story, he is forced to confront unpleasant truths about his past and his own psychological state. His investigation leads him to unearth more than just the proof he seeks, entangling him in a psychological battle against threats both external and internal. Mort's relentless pursuit for vindication draws him away from his secluded life and forces him into a corner where he must confront his demons head-on, raising questions about reality, identity, and the nature of creativity itself.