Tony Wendice, played by Ray Milland, is a former professional tennis player who has married his wife Margot, portrayed by Grace Kelly, mainly for her wealth. In a bid to make Margot happy, Tony has abandoned his athletic career and now works in the sports equipment business. However, an interesting twist arises when Margot’s past comes back to haunt them; she had once shared a romantic relationship with Mark Halliday, an American crime writer played by Robert Cummings. Despite their previous connection, Margot cut ties with Mark when he returned to the United States, leading to a long period of silence between them.
As fate would have it, when Mark finds himself back in London, Margot divulges to him that she burned nearly all of his letters, save for one that was unwittingly seized by a blackmailer who demanded a sum of £50. In a bid to reclaim the stolen letter, Margot sends the money to a pawn shop location, but her efforts are in vain as neither the letter nor the blackmailer's demands are met. Mark expresses his desire to inform Tony about their past to pave the way for Margot to divorce him, thereby reuniting with her, but Margot is paralyzed by the thought of hurting Tony's feelings. Unbeknownst to her, Tony is aware of their previous affection and harbors dark intentions.
The couple's financial entanglement deepens with both having updated their wills, designating each other as beneficiaries with a combined worth of £90,000. Over the course of a year, in a sinister preparation, Tony meticulously concocts a plan to eliminate Margot without her suspecting a thing. His plotting leads him to steal a handbag that holds a letter from Mark, orchestrating a ruse where he poses as the very blackmailer who harassed Margot, testing her willingness to pay. To ensure the plan's success, Tony meticulously tracks down an old acquaintance, Anthony Dawson, who now goes by the alias 'Captain Lesgate.' Through blackmail, Tony coerces Lesgate into agreeing to commit the murder.
Utilizing the occasion of Mark's return as an alibi, Tony orchestrates a narrative that paints a picture of a burglar’s violent intrusion. He informs Lesgate to call their home at precisely 11:00 PM, prompting Margot to answer the phone in a vulnerable situation, allowing Lesgate to strike. Meanwhile, the widow remains unaware of the orchestrated drama and is persuaded to engage in a mundane task rather than pursue her own plans.
As the setting unfolds, Margot, perhaps unknowingly participating in Tony's scheme, goes to check the phone, leaving herself exposed. However, the plan spirals into chaos when Margot, armed with incredible determination, manages to stab Lesgate in self-defense with a pair of scissors. As he falls lifelessly to the floor, Tony races to cover up his treacherous scheme, quickly reworking their narrative to frame Margot as the murderer.
In a dramatic turn, Inspector John Williams interrogates them, only to find initially plausible evidence of Margot’s guilt. The tale progresses as the courtroom drama unfolds, leading to Margot’s conviction and impending execution. Meanwhile, Tony revels in the success of his deceit, yet he is blissfully ignorant of the investigative measures that the police are undertaking, which will soon unravel his treachery.
Months later, right before Margot's scheduled hanging, Mark proposes a desperate plan to Tony—he could confess to the crime and take the fall, ultimately saving Margot's life. Coinciding events lead to the unraveling of Tony's lies, culminating in the Inspector’s cunning interception that eventually proves Tony's guilt beyond a shadow of a doubt.
Ultimately, the narrative comes full circle as Tony, flimsy layers of deception crumbling under scrutiny, realizes his meticulous web of lies is collapsing. In an ironic twist, as he stands confronted by the very people he believed he had outsmarted, he finds himself ensnared within his own malevolence, met with the gravitas of justice as Inspector Hubbard finally captures the real criminal mastermind. In the closing moments, the atmosphere grows thick with tension as tears stream down Margot's face, a symbol of both tragedy and the inevitable power of truth.
