This movie gives perfect examples of every essential element of film noir: The Venetian blinds shadowing a wall, the tall, dark main character, the femme fatale, the murder plot, Edward G. Robinson. It's all there.
The movie begins in the standard film noir style, starting us in the present, where a wounded Walter Neff, insurance agent, dictates onto a recorder his sad story of love and betrayal at the hands of one Phyllis Dietrichson. A while ago, he had stumbled upon the Dietrichson home to get a simple auto insurance renewal from the man of the house. Inside, he finds Dietrichson's wife, a none-too-subtly suggestive woman who entrances hapless Walter. He leaves to wait for Mr. Dietrichson, but comes back a few days later to see Phyllis. She lays out her plot: Her husband is in an oil field all day, where it's dangerous. She wouldn't mind having an insurance policy on him, should an . . . accident happen. Real subtle.
Walter sees through her, knows she's plotting murder, but goes along anyway. So he plans the perfect murder: Mr. Dietrichson signs off on a double-indemnity policy, thinking it's car insurance, where Phyllis would be paid double the usual rate for a certain type of accident. Second, he's set up for that little accident by being forced to take a train. Then, he happens to fall off the back of a train, breaking his neck, leaving $100,000 to his loving wife. She plays the weeping widow long enough for Walter and her to run away together.
The plan goes off without a hitch. Everything's picture perfect, except for one thing: Martin Keyes, Walter's friend and claim manager who has a "little man" in him who upsets his stomach when a claim is bad. And his "little man" is bothering him bad when it comes to the Dietrichson case. He gets closer and closer to discovering Walter, who is going stir-crazy with nervousness. When Keyes convinces himself it's murder, Walter goes to Phyllis one last time, to tell her she's found out. And he's not going down with her. At least, that's what he thinks...