Deathtrap

Playwright Sidney Bruhl debuts his latest in a series of flops and returns home. Dejected, he reveals to his wife, Myra, that he has received an excellent manuscript, Deathtrap, from an unlikely former student, Clifford Anderson, asking for Sidney's input. Cynically, he reflects that the best idea he has had lately is to murder its author and produce the play as his own.
In a telephone conversation with Clifford, Sidney invites him to their home to discuss the Deathtrap script. Sidney asks Clifford to be sure to bring all copies of the script to work on. Clifford comments that he is unable to make any copies due to his local photocopier being broken. This reassures Sidney, who comments to Myra that no copies of the script will be found after he claims authorship. To no avail, Myra tries to convince Sidney to work with Clifford as equal partners. Clifford admires Sidney's collection of stage props, and Sidney entices him to try on a pair of Harry Houdini's handcuffs, then strangles him with a chain.
Sidney removes the body, which he says he will bury on the property, but has difficulty convincing a traumatized Myra to conspire with him. When Sidney returns from burying the body, Myra insists that Sidney must buy the property from her and that they will discreetly divorce. She reveals nothing during an unexpected visit from psychic neighbor Helga Ten Dorp. Helga senses pain and death in the house, warning Sidney about a man in boots who will attack him.
As the Bruhls prepare for bed, a mud-spattered Clifford bursts through the bedroom window and beats Sidney with a log, then chases Myra through the house until her weak heart gives out and she dies.
Descending the stairs, uninjured, Sidney joins Clifford. They discuss what to do with Myra's body, then kiss. The previous events have been a ruse to kill Myra, the wealthy spouse, without laying a hand on her. Clifford moves in and the two work on separate literary projects at a partner's desk. Sidney suffers writer's block, but Clifford types page after page of a play that he keeps under lock and key.
Sidney does not trust Clifford, a sociopath with an extensive criminal record; when Clifford is out of the room, he switches Clifford's manuscript with a fake. Sidney is horrified to read that Clifford is using Myra's murder as the basis of a new play called Deathtrap. He confronts Clifford, who boasts about the play's potential and insists he will write it with or without Sidney's approval. Sidney plays along with collaborating while he plots a solution. Helga stops by for candles in anticipation of a thunderstorm, warning Sidney that Clifford is the man in boots.
To demonstrate a plot point, Sidney asks Clifford to arm himself with an axe, then produces a gun. He intends to shoot Clifford, claim it was in self-defense, and dispose of the Deathtrap manuscript. The gun is empty. In anticipation, Clifford had loaded the bullets into a different gun. Manacling Sidney to a chair, Clifford reveals that he intentionally flagged his locked manuscript, which he left unattended so Sidney would read it and give him the third act to his play by his actions. He warns Sidney not to try to stop the production of the play.
Clifford is unaware the manacles are trick shackles. Sidney releases himself, grabs a crossbow, and shoots Clifford in the back. The storm hits and the house suffers a blackout. A flash of lightning illuminates Helga scurrying through the room, thinking Sidney is in danger and coming to help. She realizes it is Sidney that actually poses the threat and grabs a gun while he finds a knife. Clifford regains consciousness and trips Helga. A struggle for the gun ensues. Clifford swings the axe at Sidney.
In that very moment, the scene transitions to a stage version of itself, with actors before a full house. The onstage struggle culminates with "Clifford" stabbing "Sidney" and both dying, leaving "Helga" victorious. The opening night audience erupts in applause. At the back of the theater stands an exultant Helga Ten Dorp, author of hit Broadway play Deathtrap.