In 1938, Walter Neff, an insurance salesman, returns to his office in downtown Los Angeles late one night. In pain from a gunshot wound to his shoulder, he begins dictating a confession into a dictaphone for his friend and colleague, Barton Keyes, a claims adjuster. The story, told primarily in flashback, ensues.
Neff meets the alluring Phyllis Dietrichson during a house call to remind her husband to renew his automobile insurance policy. They flirt, until Phyllis asks about taking out a policy on her husband’s life without his knowledge. Neff deduces she is contemplating murder, and initially wants no part of it, but eventually devises a plan to murder her husband and trigger the “double indemnity” clause, which would double the payout.
Phyllis drives her husband to the train station for a trip to attend a college reunion. Neff, hiding in the back seat, breaks his neck. Neff then boards the train posing as Mr Dietrichson and makes his way to the back of the train, stepping onto the platform outside to smoke a cigarette. He jumps off at a prearranged spot to meet Phyllis and drag Dietrichson’s body onto the tracks.
Mr. Norton, the company’s chief, believes the death was suicide, but Keyes scoffs at the idea. Keyes suspects foul play on Phyllis’ part because he thinks she was having an affair. Norton refuses to pay out the accidental death clause, deducing that Dietrichson was unaware of the policy. Keyes tells Neff his theory while Phyllis is hiding behind the door, concluding that Phyllis and an accomplice murdered Dietrichson for the insurance money, but he needs proof.
The victim’s daughter, Lola, tells Neff she is convinced that her stepmother Phyllis is behind her father’s death. Lola’s mother also died under suspicious circumstances when Phyllis was her nurse. Neff begins seeing Lola to keep her from going to the police with her suspicions, and later through guilt to protect her from Phyllis. Neff fears Phyllis will murder Lola over her suspicion in her parents’ murders and because Dietrichson had changed his will in Lola’s favour, leaving Phyllis with nothing.
Keyes brings in a witness who says the man on the train was younger than the dead man. Neff warns Phyllis that pursuing the insurance claim in court risks exposing the murder. He points out that Norton and Keys are suspicious of her. He tries to convince Phyllis to lie low and let him try to convince Norton to pay out the claim. Lola tells Neff she has discovered that her boyfriend, the hotheaded Nino, has been seeing Phyllis behind their backs. Neff sees this as a way to get rid of Phyllis before she attempts to murder Lola.
Neff meets Phyllis at her house and informs her that he knows about her and Nino. He guesses she is planning for Nino to kill him, but tells her that he intends to kill her and put the blame on Nino. She shoots him in the shoulder. He dares her to shoot again. She does not, and he takes the gun. She says she never loved him “until a minute ago, when I couldn’t fire that second shot.” As she hugs him, Neff says “Goodbye, baby,” and shoots twice, killing her.
Outside, Neff waits for Nino and advises him not to enter the house. He convinces Nino to instead telephone Lola. Neff drives to his office and starts speaking into his dictaphone, returning to the start of the film. Keyes arrives unnoticed and hears the truth. Neff tells him he is fleeing to Mexico, but he is too weak and collapses. Keyes lights Neff’s cigarette as they wait for the police and an ambulance.