Psychotherapist Alan Strauss is grieving the recent death of his wife when he wakes up chained to a bed in an unfamiliar basement. His captor is Sam Fortner, a seemingly mild-mannered patient who revealed in therapy that he is a serial killer. Sam confesses to Alan that he desperately wants to stop murdering, believing that Alan is the only therapist skilled enough to cure his homicidal compulsions. Recognizing the extreme peril he is in, Alan realizes his survival depends entirely on successfully treating a psychopath who refuses to stop killing entirely.Confined to the room, Alan engages in intense, psychological sessions with Sam, navigating a minefield of threats while trying to unlock the root causes of Sam’s violent urges. Alan traces Sam’s trauma back to an abusive,
domineering mother, Candace, who lives upstairs and remains oblivious to the horrors happening beneath her feet. As the days bleed together, Alan grapples with profound isolation and survivor's guilt, frequently hallucinating conversations with his late wife, Beth, and his former mentor, Charlie. These spectral counselors offer
emotional support and debate the ethics and futility of his predicament.Desperate to escape before Sam inevitably kills again, Alan attempts various survival strategies. He tries to subtly manipulate Sam into turning himself in and even attempts to drive a wedge between Sam and Candace. However, these psychological maneuvers repeatedly fail against Sam’s rigid, unyielding pathology. When Sam snaps and murders an abusive boss in a renewed killing spree, Alan realizes standard therapy is failing. In a final, harrowing gamble to save innocent lives and secure his own freedom, Alan decides to play a deeper psychological card.Knowing Sam views him as a father figure, Alan writes a poignant, brutally honest letter detailing his legacy and feelings, intending to force Sam to confront the immense pain he inflicts. However, Sam interprets the gesture as a final betrayal and ultimate rejection. Recognizing that Sam will never let him leave alive because he knows too much, Alan attempts a violent, last-ditch escape using a heavy object to attack his captor. The attempt is tragically futile; Sam overpowers him and, in a blind rage, kills Alan.Months later, the basement is raided by law enforcement. A harrowing post-credits scene reveals the tragic legacy of the ordeal: Alan’s estranged son, Ezra, finds the hidden video tape Alan recorded for him while chained, finally achieving a measure of emotional reconciliation posthumously. Meanwhile, Sam is fitted with an electronic tracking anklet, having chosen to chain *himself* up in his mother’s basement to protect the world from his incurable darkness, forever imprisoned by his own unyielding mind.