On a dark Los Angeles night in July 1938, insurance agent Walter Neff is bleeding from a gunshot wound and slips into his office at the Pacific All Risk Insurance Co. Walter records his murder confession on the dictaphone, addressing his boss and friend, Barton Keyes, a meticulous and intuitive claims agent. Walter thinks back to May when it all started: Walter visits an expensive Spanish-style house in Los Feliz to follow-up an automobile insurance renewal for Mr. Dietrichson. He is immediately attracted to Dietrichson's wife Phyllis, who first appears clad only in a towel. Walter flirts with Phyllis, whose interest is piqued, nevertheless, she rebuffs him and the next day changes his appointment to meet with her husband. When Walter arrives that day, he and Phyllis are alone and she inquires about getting an accident policy for her husband without his knowledge. Upset by her implications, Walter leaves, but his expectation that he has not seen the last of Phyllis is fulfilled when she appears at his apartment. Walter soon gives in to his longing and kisses Phyllis, after which she reveals that she has been abused and neglected by her husband. Phyllis admits to having fantasies of killing Dietrichson, but his life insurance beneficiary is his mature daughter Lola, who hates her. Walter is repulsed by, and at the same time, strangely drawn to Phyllis's fantasy, and his thoughts linger on how to accomplish an undetectable crime. Agreeing to help Phyllis kill her husband, Walter meets with Dietrichson and, in Lola's presence, tries to sell him accident insurance. Dietrichson refuses the accident insurance, but enrolls for auto insurance, and is unaware that Walter has given him an accident insurance form to sign as well. Walter secretly advises Phyllis to book a train for Dietrichson's business trip, as a double indemnity clause in the policy will award her double the stated $50,000 if Dietrichson dies from an unlikely cause, such as a train accident. Phyllis and Walter begin to meet surreptitiously every morning in a local market. Dietrichson breaks his leg just after the accident policy comes through, and the lovers are delayed in carrying out their plan. In mid-June, as Keyes offers to hire Walter as his assistant, Phyllis telephones and informs Walter that Dietrichson is leaving that night on the train. Walter turns down Keyes' offer and after leaving the office, calculates his every move to avoid future suspicion, then hides in the Dietrichsons' car. After Phyllis uses a pre-arranged signal, Walter sits up from the back seat and strangles Dietrichson to death. Dressed as Dietrichson, Walter then boards the train and heads for the observation car. Walter is dismayed to find another passenger, Jackson, sitting on the deck, but when he leaves to get Walter a cigar, Walter jumps off the back of the train. After leaving Dietrichson's body on the tracks, Phyllis and Walter leave together in her car. The police declare Dietrichson's death accidental, but Norton, the president of All Risk, is reluctant to pay out the $100,000 and meets with Phyllis. Phyllis pretends to be bereaved and is genuinely shocked at Norton's suggestion of suicide. After she leaves, Walter is delighted when Keyes assures Norton that he will have to pay out the claim. At his apartment later that night, Walter is surprised by a visit from Keyes, who has developed indigestion due to an incongruity in the case: Dietrichson never filed a claim for his broken leg, even though he had just purchased accident insurance, in addition to which, the train was going so slowly that suicide is unlikely. Keyes concludes that Dietrichson was ignorant of the policy, and he is suspicious of Phyllis. A nervous Walter rushes Keyes out, as Phyllis hides behind the door to escape notice. The next day, Lola confides in Walter that she suspects that Phyllis, who was her mother's nurse, killed her mother six years earlier and now has done the same to her father. In order to distract Lola, Walter spends the next few days with her, and learns that she has broken up with her college drop-out boyfriend, Nino Zachette. During this time, Keyes becomes convinced that Dietrichson was murdered, and sends for Jackson. Jackson confirms that the man on the train does not match photographs of Dietrichson, and Keyes subsequently has Phyllis followed by detectives. Walter urges Phyllis not to sue for the claim, which is now being withheld, as Keyes will oppose it, but she is determined to get the money, and insists that the murder was all his doing. Walter is now suspicious of Phyllis, as Lola has told him that Nino is seeing her stepmother, and Walter thinks about killing her. Phyllis files suit for the insurance money, and Keyes tells Walter that her partner-in-crime has shown himself. Worried that Keyes is on to him, Walter listens to Keyes's dictaphone and hears that Keyes suspects that Nino is Phyllis' partner-in-crime, after which he arranges to meet with Phyllis late that night. Unknown to Walter, Phyllis has prepared for his visit by hiding a gun under a seat cushion. Walter confronts Phyllis and tells her that he knows she has used him and that he intends to frame Nino for the murder. Phyllis then shoots Walter, but is unable to kill him. Admitting that she has never loved him, Phyllis now embraces him, and Walter shoots her twice, killing her. As he leaves the house, Nino walks up, and Walter urges him to go to Lola, who truly loves him. By 4:30 a.m., Walter finishes his confession as Keyes makes his presence known, having been called by the janitor who noticed Walter trailing blood. Walter walks out, intending to escape to the border, but collapses before he gets to the elevator. Keyes, disappointed, nevertheless reveals his affection for Walter, and Walter reciprocates, as Keyes lights Walter's final cigarette.