Sidney Bruhl, portrayed by Michael Caine, is a once-renowned playwright known for his suspenseful production The Murder Game. After his latest play fails to impress, he retreats to his home in East Hampton, accompanied by his wife, Myra (Cannon). In a twist of fate, Sidney receives a new script titled Deathtrap from a former pupil, Clifford Anderson, played by Christopher Reeve. Initially jesting about murdering the young writer to steal his work, the situation grows grim when Sidney discovers that Deathtrap is unique; no one else has read it.
As Clifford arrives in Long Island, Myra becomes increasingly anxious, urging Sidney to collaborate with him and share the potential profits from the play. However, Sidney's desperation takes a dark turn as he violently attacks Clifford, leading to her horrified compliance in disposing of the body. After burying Clifford, an eerie twist occurs with the arrival of psychic Helga ten Dorp (Worth), who perceives death and strife within their home. She warns Sidney of great danger, sensing a looming threat from a “man in boots.”
In a shocking turn of events, Clifford reappears, having survived the attack, and unleashes his vengeance on Sidney, further complicating the already fraught situation. As Myra’s health deteriorates under the stress, she collapses and dies, leaving Sidney unperturbed, yet the tide turns in unexpected ways as tension escalates between him and Clifford. The two share a passionate kiss, marking a pivotal moment in their twisted bond—a grim culmination of Sidney's deeper ambitions as Myra’s presence fades from their lives.
After the funeral of Myra, Clifford settles into Sidney’s home as his "secretary," working on a controversial play that recounts the events of Myra's murder. As Sidney grapples with writer's block, he becomes increasingly paranoid about Clifford’s intentions. When Sidney's lawyer, Porter (Jones), comes to deal with Myra’s affairs, he notices Clifford's peculiar behavior surrounding his manuscript. Driven by a mix of jealousy and ambition, Sidney clandestinely reads Clifford’s script, uncovering the chilling realization that his young protégé is penning the true story of Myra's demise.
A tense conflict brews between the two creators when Sidney confronts Clifford, leading to a struggle for recognition that culminates in violence and betrayal. Adding to the ominous atmosphere, Helga returns, sensing another imminent threat as a storm brews outside. Sidney, in a desperate quest to eliminate any threats to his reputation, proposes a rehearsal that spirals into a deadly confrontation involving firearms, forced confessions, and elaborate entrapments as he seeks to regain control over the narrative of his life.
In this sequence of betrayals and miscalculations, the power dynamics flip as Sidney finds himself in a life-or-death situation, chained and vulnerable. Clifford seizes the opportunity, embodying an unsettling blend of cunning and ambition as he takes command. However, Sidney subtly outsmarts his captor, resulting in a tragic turn of events that leaves both characters grappling with their fates.
With dark humor and grim irony, the narrative unfolds to a chaotic climax where the characters' struggles are exposed in a theatrical spectacle that blurs reality and fiction. The ending sees a shocking spotlight shine on Helga, who emerges victorious, transforming the harrowing tale of murder and betrayal into a celebrated Broadway hit titled Deathtrap. The audience’s thunderous applause echoes the twisted legacy of Sidney's ambition, while the specter of loss lingers in the aftermath of betrayal and ambition gone awry.
