In 1938, Walter Neff, an experienced salesman of the Pacific All Risk Insurance Co., meets the seductive wife of one of his clients, Phyllis Dietrichson, and they have an affair. Phyllis proposes to kill her husband to receive the proceeds of an accident insurance policy and Walter devises a scheme to receive twice the amount based on a double indemnity clause. When Mr. Dietrichson is found dead on a train-track, the police accept the determination of accidental death. However, the insurance analyst and Walter's best friend Barton Keyes does not buy the story and suspects that Phyllis has murdered her husband with the help of another man.

When Nick Parsons appears to be murdered his wife Libby is tried and convicted. Six years later Libby is paroled and with the help of Travis Lehman (her parole officer) she sets out to find her son and the truth behind the "murder".