After poisoning and killing his wife, the master of the house, Stephen Lowry, Stewart Granger is blackmailed by his Cockney maid, Lily Watkins, who demands a promotion and the chance to reshape their fate. As Lily steadily takes the place of his dead wife, Lowry’s fear of exposure grows, and he becomes increasingly desperate to silence her as well. In a tense attempt to wipe away the threat, he trails a look-alike through the fog, hoping to mislead any pursuit, but the plan spirals out of control when he ends up killing the wife of Constable Burke and triggering a chaotic chase by an angry mob that he narrowly dodges.
Lily returns home, and Stephen learns of this dangerous misunderstanding. Word spreads through the locals that Lowry killed Mrs Burke, and he is soon put on trial. Yet the proceedings are muddled when the bar-goers’ testimonies are dismissed after it’s exposed that they drink heavily, and Lily crafts a clean alibi by lying about her own lack of movement. The strongest testimony, however, is Lily’s own claim that Stephen never left the house, a lie she tells to secure a future with him.
Behind the drama, Lowry’s heart pulls toward Elizabeth Travers, the daughter of a wealthy man and a desired prize of his lawyer. He hints to Lily that this is all part of a larger plot to gain money, and he envisions using that fortune to take Lily with him to America. He even demands that Lily retrieve a letter she sent to her sister, which could expose his actions. The incriminating letter is salvaged by her sister’s husband, Herbert Moresby William Hartnell, who carries it to Lowry’s lawyer and tries to extort £500 for its return.
Lowry, not content to wait for justice, feigns illness and sends Lily to fetch the doctor. He calculates that the five-minute window will be enough to frame her by ingesting the very poison he used on his wife and by planting it, along with the jewelry, in Lily’s room. The plan is almost perfect, but Lily is detained by the police when the “tell-all” letter surfaces, threatening to derail his scheme.
The plan blows up in Lowry’s face as he grows weaker and realizes his grip on power is slipping. He enlists Constable Burke to fetch the doctor, hoping a swift intervention will salvage his innocence. Meanwhile, Lily’s handwriting is examined against the letter; at first she’s told the handwriting doesn’t match, but the evidence ultimately confirms she was implicated in a larger conspiracy. A warrant is sworn for Lowry’s arrest, and Lily returns to the house to piece together the events, discovering that Stephen never loved her as he claimed and that his accusations of poisoning were a cruel manipulation.
As the doctor pronounces that it is too late to save him, Lowry confesses with the chilling line, > I timed it, you said you'd be only five minutes! The confession seals his fate, and Stephen dies, leaving Lily to face the next phase of justice with the police.
