William Marble Charles Laughton is a London bank clerk who finds himself drowning in debt and staring down the threat of dismissal when his boss learns of an overdue bill. On the same day, a coworker drops a tantalizing currency-market tip that promises big gains if only Marble had money to risk on the market. The situation spirals as Marble’s prospects hinge on access to cash he doesn’t have, and the pressure mounts.
That evening, a wealthy Australian nephew, James Medland Ray Milland arrives in Marble’s life. Marble spends hours trying to persuade Medland to lend him the money needed to invest on the tip or, at the very least, to extend a loan. Medland, however, is uneasy and resistant, sensing trouble beneath Marble’s smooth talk and anxious explanations. Desperation pushes Marble toward a lethal step: he offers Medland a glass of whisky laced with cyanide that he had bought previously for developing photographs, and under the cover of darkness he buries his rival in the back yard.
With Medland out of the way, Marble inherits the late man’s cash and immediately places a margin bet on the tip. The gamble pays off handsomely, yielding £30,000 and enabling Marble to retire from the grind of his daily job. Yet the windfall does little to steady his nerves. He becomes tense, irritable, and increasingly secretive, refusing requests to move to a larger, more comfortable home. He turns to books about crime and poisons, carefully rereading a page that describes cyanide, as if rehearsing a script for what he has done.
His wife, Annie Marble Dorothy Peterson, senses that something is terribly wrong but accuses him of embezzling from the bank rather than acknowledging a crime of murder. To ease his mounting tension, Marble sends Annie and their daughter, Winnie Marble Maureen O'Sullivan, away on a three-week vacation. While they are gone, a local shopowner, Rita Collins Verree Teasdale, seduces Marble to borrow £300, smoothing over the immediate financial crunch but deepening the moral rot at the core of Marble’s life. When Winnie returns a day early, she stumbles upon Collins in the house but chooses to keep quiet about what she has seen.
Even as Marble’s new wealth grows, misfortune follows him. Annie notices a newspaper ad seeking Medland and, in a moment of alarm, glimpses the page bearing the cyanide motif in Marble’s notebook—she realizes what he has done but remains loyal to him, at least for the moment. Winnie begins to drift into a snobbish social orbit, looking down on her parents and their new wealth. After a late-night run in the rain, Annie falls gravely ill, only to be saved by Marble’s careful care and waitful concern, which brings a fragile sense of relief to the family.
Yet trouble returns with a vengeance when Collins reappears and uses his influence to extract a further £500 from Marble, edging the family deeper into danger. Annie overhears the details of the affair and, overwhelmed by despair, ends her life with more of the cyanide Marble had purchased. The authorities indict Marble for Annie’s murder, and he is eventually convicted for her death.
In the final scenes, Winnie visits Marble on the day of his execution, overcome with grief and remorse. He reassures her that he did not kill Annie, but he accepts his fate with a quiet peace, suggesting that he sees his crime as a bill he has paid, albeit a bill that was long overdue.
