An insurance salesman, Walter Neff, becomes infatuated with the seductive wife of one of his clients, Phyllis Dietrichson. Phyllis, an unhappily married woman, sees an opportunity in Walter's attraction to her. She persuades him to concoct a plan that will rid her of her domineering husband and allow them to claim a hefty life insurance payout. The scheme revolves around a "double indemnity" clause that promises a doubled payout in cases of accidental death under certain conditions. The pair meticulously plan what they believe is the perfect murder, designed to look like an accidental fall from a train, ensuring that it meets the criteria for the double payout.

As the duo puts their plan into motion, Walter's own company's insurance investigator, Barton Keyes, starts to suspect foul play. Keyes is a shrewd and tenacious examiner with an instinct for detecting insurance fraud, and he becomes increasingly convinced that the accidental death was orchestrated. Meanwhile, the relationship between Walter and Phyllis begins to strain under the pressure of their guilt and paranoia. The partners in crime start to mistrust each other and the meticulous plans they have crafted begin to unravel as the investigation draws closer to the truth.