1938. Los Angeles. The night is dark and the blood seeping from Walter Neff is red. Neff is an insurance salesman for the Pacific All Risk Insurance Co. and he is in his office, recording a confession into his Dictaphone. The confession is addressed to Barton Keyes, the claims agent who is his immediate superior. It all started, he begins, on that day in May when he headed out to the Dietrichson home to update their automobile coverage.
Mr. Dietrichson is not at home when Walter arrives. Instead, he is greeted by Mrs. Phyllis Dietrichson, a sexually enticing woman with whom Walter flirts. The absence of her husband continues when Walter tries again to meet with him, and the sexual tension ramps up. When Phyllis makes some inquiries about the possibility of taking out an accident insurance policy on her husband without her husband’s knowledge, however, Walter gets spooked by the implications of murder and takes his leave.
Even so, he can’t get Phyllis completely out of his mind, and he is not surprised when she suddenly shows up at his apartment. She admits that her husband has abused and essentially abandoned her, and that she has fantasized about being rid of him. They both give in to the rising passion and embrace and kiss. Walter learns that the beneficiary of Mr. Dietrichson’s life insurance is not Phyllis, but his daughter Lola. Lola and Phyllis despise each other. Against his better judgment—which is about to almost completely wither under the influence of his unwise passion—Walter mulls over how to get around this obstruction. Finally, he agrees to help Phyllis off the husband.
Walter finally meets with Mr. Dietrichson and with Lola present attempts to sell him on the idea of accident insurance. He refuses but accepts the wisdom of purchasing auto insurance, but Walter furtively slips the accident form in for him to sign as well. The policy that Mr. Dietrichson just unwitting signed carries a double indemnity clause which pays off double the face value in the event of death from a rare accident such as falling off a train. Phyllis and Walter make arrangements for it to appear as if Mr. Dietrichson fell from a train so Phyllis can collect the $100,000.
The plan is not carried out until June, after the claims agent Barton Keyes has extended an offer to make Walter his assistant. Walter turns down that offer and puts the plan into action. With Walter hiding in the back seat, Phyllis drives her husband to the train station, but before he gets there Walter emerges and strangles him. Walter then disguises himself as Mr. Dietrichson and boards the train. His plan to simply jump off the caboose is interrupted by the unexpected sight of another passenger. When the passenger heads back into the train to get "Mr. Dietrichson" (i.e. Walter) a cigar because the poor man is hobbling around on crutches, Walter jumps over the side. They place the dead body on the tracks and leave by car.
While the police have ruled that the death was an accident, the President of Walter’s insurance company is suspicious and reluctant to shell out the $100,000. He meets with Phyllis who is authentically taken aback by the suggestion that she may not be deserving of a payout because her husband’s death may have a suicide. After she leaves, Keyes makes it clear that the payout will have to be made because of the statistical improbability of it being a suicide.
Later that night, Keyes shows up at Walter’s apartment with a gut feeling that something is not right. Dietrichson supposed had a broken leg, yet never filed a claim for it despite having just purchased accident insurance. Keyes dismisses the husband as the culprit for something shady taking place, but not Phyllis. As Keyes is leaving, Phyllis shows up and has to hide behind the open door and sneak in without being noticed.
The very next day, Lola asserts her suspicion of Phyllis, and Walter learns the cause of their mutual hatred: Phyllis was hired to nurse her mother, who then died mysteriously while in the care of Phyllis—who would, of course, go on to marry her father. Lola suggests that Phyllis is responsible for the death of both of her parents and worries she may be next.
In the meantime, Keyes has now concluded that Dietrichson was murdered and locates the man who was on the platform with him just before he supposed fell over onto the tracks. After confirming that the man he saw looks nothing like the real Dietrichson, Walter advises Phyllis not to sue for her money which is being withheld pending investigation. Lola informs Walter that her boyfriend Nino is also romantically involved with Phyllis, and a betrayed Walter ponders the idea of killing her.
Phyllis goes ahead and files the suit to get the money, to which Keyes responds by telling Walter that her partner in the murder has just revealed himself. Paranoid that Keyes knows the whole story, he listens to the recording on Keyes’ Dictaphone and learns that Nino is the suspected partner. Walter heads to meet with Phyllis unaware that she is waiting for him with a concealed gun. Water tells Phyllis that he knows she only used him and is going to frame an innocent Nino. Phyllis grabs the gun and shoots Walter. Phyllis embraces him and Walter grabs the gun and shoots twice, killing Phyllis. He struggles to make his way out of the house and runs into Nino, telling him to go back to Lola as she is the one who really loves him.
It is 4:30 in the morning back in Walter's office, as his confession into the recording device concludes and Keyes appears. Walter leaves Keyes standing there, thinking he can make it to the border, but collapsing even before he can make it to the elevator. Keyes lets Walter know he is disappointed, but still has great affection for the younger man. Walter lets Keyes knows the feeling is mutual and accepts one final cigarette before dying.