Composer Norman Martell is having an affair with Peggy, half sister of his wife, Helen. Driving Peggy home after a party, he knocks down and injures a man on a country lane. Despite Peggy's protests, he refuses to stop at the scene of the accident, and does not report it.

Some time later, Peggy reveals that she is pregnant with Norman's baby and insists that he tell Helen of their affair. She demands that he leave his wife to be with her. Reluctantly, Norman agrees and does this, but Helen (who, unknown to Norman has a serious heart problem) refuses to give Norman a divorce.

Norman enlists Peggy's help and together they plan to murder Helen, constructing what they think will be a watertight alibi for the crime, to be provided using a recording of Norman's voice on a state-of-the-art tape recorder, which will be played by Peggy during a telephone call at a certain time. However, unknown to the pair, Helen overhears the plot.

Norman and Peggy attempt to carry out their scheme, but, because Helen knows of the scheme, all does not go according to plan. Ultimately, Helen shoots Peggy. During the consequent police investigation, Helen suffers a fatal seizure. With Helen dead, Norman is implicated in Peggy's murder. He tries to clear himself, but it transpires that the only man who could have testified to his innocence was the man Norman knocked down in his car. Blind since the hit-and-run accident, he is unable to back Norman up.