British playwright Sidney Bruhl leaves the theater upon hearing disapproving whispers from the audience during the opening night performance of his Broadway play, Murder Most Fair. He enters the restaurant lobby next door and telephones his sickly wife, Myra, who is home in bed. Despite her reassuring words, Sidney gets drunk while watching television critics negatively review his play. He boards a train home to East Hampton, New York, but falls asleep and misses his stop, causing him to take an expensive taxi ride back to his house. When Myra sees her husband in the living room, she screams in surprise and Sidney drops his drink. Agitated, he exclaims that Murder Most Fair marks his fourth consecutive failed production, but Myra reminds him that his play The Murder Game remains the longest-running thriller in Broadway history. Sidney reports that he has received a copy of an amateur, but excellently-written, play titled Deathtrap from one of his former seminar students, Clifford Anderson. Later, after Sidney has rested, he jokingly suggests that he kill Clifford with the mace that hangs among a collection of prop weapons used in his previous plays, and debut Deathtrap under his own name. As Sidney laments his failing career, Myra encourages him to produce the play instead. She then mentions Helga ten Dorp, a Dutch psychic who is renting the neighbor’s house next door. Myra urges her husband to telephone Clifford, but he becomes absorbed with his murderous plot, enticed by the play’s box office potential. He telephones Clifford to compliment him on his work and invites him to the house that evening, requesting that the young playwright bring the original draft and all additional notes and copies. Sidney informs Myra that while Clifford is in town house-sitting, he has no car, no companions, no wife, and no one else to know that he is working on Deathtrap. That evening, Sidney picks Clifford up at the train station and brings him home, where the younger man admires the Bruhls’ collection of prop weapons. While the two men discuss Deathtrap, Myra proposes that Sidney collaborate with Clifford as a producing partner. The younger man refuses to make a rash decision, however, and suggests he speak to an agent first. As Clifford prepares to leave, Sidney shows him a pair of prop handcuffs on his wall. When Clifford puts the cuffs on, Sidney pretends to search for the lost key, and Clifford grows nervous. The young man states that he is expecting a phone call from a girl who knows he is visiting the Bruhl household. When Sidney detects Clifford’s lie, Myra believes that her husband is about to kill his protégé. However, Sidney finds the key, and the three laugh to break the tension. As Clifford attempts to free himself, Sidney strangles him with a chain. Sidney then cleans the weapon, and burns Clifford’s original notes and drafts. After he and Myra bury the body in the vegetable garden outside, Myra expresses her disgust. Although she believes they will be caught, Sidney invents various lies to tell the police and promises to donate half of the play’s profits to a dramatic academy. Suddenly, the doorbell rings and Helga ten Dorp enters the kitchen, guided by a psychic vision. She walks through the house and announces that she can sense the physical pain that has occurred there that evening. Picking up a knife that was used in The Murder Game, she predicts that a woman will use it again. She then envisions a young man wearing boots, whom she believes will attack Sidney. Before bed that night, Myra is frightened by a creaking noise in the living room. Sidney forces her to go downstairs, and assures her of her safety. When he claims full responsibility for the murder, the couple retreats to their bedroom. As Myra opens the window, Clifford, covered in blood and dirt, jumps into the room, beats Sidney with a tree branch, and chases Myra downstairs, where she dies of a heart attack. Sidney joins Clifford downstairs and reveals that, although they had rehearsed the entire scenario, Sidney had not planned to kill Myra. The two men kiss, and Sidney burns the copy of Deathtrap, advising Clifford to hide in the bedroom while he reports Myra’s death to the police. Some weeks after Myra’s funeral, the two men live together as lovers and writing partners: Sidney suffers from writer’s block, while Clifford types up a new play, which he claims is a departure from his usual work in the thriller genre. When Sidney’s friend and attorney, Porter Milgrim, stops by, Clifford excuses himself to pick up groceries. Sidney explains that he has hired Clifford as his secretary, and says that he suspects Clifford to be homosexual. Porter informs Sidney of the money he will receive from Myra’s death, which the playwright exclaims to be a much higher sum than he had anticipated. As Porter leaves, he informs Sidney that he noticed Clifford suspiciously locking his work in the desk drawer. Sidney attempts to open the drawer, but is thwarted by Clifford’s return. Once Clifford unlocks the drawer, Sidney switches his lover’s documents with his own. As Sidney reads, he discovers that Clifford is actually writing another draft of Deathtrap, containing a plot mirroring their scheme which killed Myra, with many of the details preserved. Distraught by the possibility of being caught, Sidney threatens to burn the pages, but Clifford insists that even if the play’s debut does raise suspicion, there is no quantifiable evidence that could put Sidney in jail. Additionally, his status as a man who got away with murder could enhance his celebrity and increase ticket sales. Despite hesitating to trust Clifford, Sidney admits that his inheritance from Myra is disappointingly insufficient, and agrees to help Clifford write the play for the money. Clifford expresses concern about the action of the second act of Deathtrap, but Sidney promises to think about possible edits. During a thunderstorm that night, Helga ten Dorp visits the house while Sidney is out to dinner. When Sidney arrives home, she warns him that Clifford is the man in boots who will attack him. Sidney says that he planned to fire Clifford anyway, and she leaves, comforted by the lie. While Clifford is upstairs, Sidney takes one of his guns from the wall and hides it on the mantelpiece. As Clifford and Sidney act out a scene from the play, Sidney points his gun at Clifford, explaining that his inheritance and insurance money actually total nearly $1 million. Reciting the story that he will tell the police, he pulls the trigger, but the gun does not fire. Clifford pulls another gun off the wall and reveals that the copy of Deathtrap Sidney read was a decoy. He commands Sidney to handcuff himself to a chair as he recites the action of the play’s final act, in which the protagonist, modeled after Sidney, commits suicide. Clifford says he will leave and finish Deathtrap himself, instructing Sidney to never interfere. Sidney breaks free from his handcuffs, grabs a crossbow, and climbs onto the roof and into the upstairs bedroom. Descending to the living room, he shoots Clifford in the back, pulls the spear from his lover’s body, and manipulates the scene so it appears as if Clifford had been attacking him. When the power goes out, Helga ten Dorp appears in the house and points Sidney’s prop gun at him. Reviving, Clifford grabs Helga’s ankle, tripping her, pulls himself up, and swings an axe at Sidney. As actors recreate the final scene onstage during the opening night performance of Deathtrap, the audience roars with applause and Helga ten Dorp, the play’s writer, jumps with glee at her success.