After poisoning his wife, the master of the house Stephen Lowry (Stewart Granger) is blackmailed by his Cockney maid Lily Watkins (Jean Simmons), who demands promotion. As she steadily takes the place of his dead wife, he again attempts murder. While attempting to murder Lily, by following someone who looked like her through the fog, he mistakenly kills Constable Burke's wife and gets chased by an angry mob, which he evades. Lily returns home and Stephen learns of his mistake. Some local bar goers saw him murder Mrs Burke and Stephen is put on trial, but their claims are dismissed after it is revealed they drink a lot and Lily lies to provide an alibi.