Frank Chambers (John Garfield) is an amiable, restless drifter who has hitched a ride with a man we later learn is the local District Attorney, Kyle Sackett (Leon Ames). He drops Frank off at a rural diner/service station on a highway in the hills outside Los Angeles, Twin Oaks. Frank ends up working there. The diner is operated by the stodgy Nick Smith (Cecil Kellaway) and his beautiful, much younger wife, Cora (Lana Turner).
Frank and Cora start to have an affair soon after they meet. Cora is tired of her situation, married to a man she does not love and working at a diner that she wishes to own outright. During an attempt to run away together, Cora concludes that if she divorces Nick, she will end up with nothing; she and Frank will be no further ahead. They return to Twin Oaks in time for her to retrieve the goodbye note she had left in the cash register for Nick. Cora talks Frank into murdering Nick, in order for them to have the diner. The plan involves Cora striking Nick with a sock full of ball-bearings and pretending he had fatally hit his head falling in the bathtub. Things go awry when a police officer stops by and a cat causes a power outage. Cora does manage to knock Nick over the head and, while severely injuring him, he is not mortally wounded.
The couple are thrilled when it is determined that Nick will be all right, since no foul play is suspected, and that he has no recollection of how he was struck. They share a happy week, running the business together and enjoying their relationship. The police officer stops by one day and tells Frank he passed Cora driving Nick back from the hospital. Frank sees there is really no hope for a definite future with her, so he decides to move on before she returns. He goes to L.A.; after a couple of weeks, he starts hanging around the marketplace where Nick and Cora buy most of their produce, hoping to see her. He runs into Nick, who has been looking for him; Nick insists Frank return to Twin Oaks with him, that “something important’s gonna happen tonight and you’re in on it”.
Upon Frank’s return, Cora behaves coolly toward him; the three of them have dinner together and Nick announces that he will be selling Twin Oaks and that Cora and he will be moving in with his infirm sister in the town of Kugluktuk in northern Canada. That night, Cora is desperate; Frank finds her in the kitchen with a knife she says she was going to use on herself. Frank agrees to kill Nick. The next day, the three of them are to drive to Santa Barbara to finalize the sale of Twin Oaks. Frank and Cora intend to stage a drunk driving accident. Sackett stops by for gas and Frank and Cora stage an argument where she insists on driving due to the men’s inebriation. This establishes that Nick is drunk. On a deserted stretch of road, Frank kills Nick with a blow to the head and then sends the car off a cliff. However, Frank is caught in the car too and injured. Sackett, who had been following them, arrives to find Cora crying for help.
The District Attorney files murder charges against only Cora, hoping to divide her and Frank. Although this ploy works temporarily, a clever measure by Cora’s lawyer, Arthur Keats, (Hume Cronyn) prevents Cora’s full confession from coming into the hands of the prosecutor. Cora secures a plea bargain in which she pleads guilty to manslaughter and receives probation.
Publicity from the murder makes Twin Oaks very successful, but things remain tense between Frank and Cora. They marry in order to protect themselves from being forced to testify against each other. When Cora leaves to take care of her sick mother, Frank has a brief fling with a woman. After Cora returns, a man named Kennedy, who had worked as investigator for her attorney, shows up and attempts to blackmail her with the confession. Frank beats up Kennedy and his partner and takes the signed confession from them.
Cora tells Frank that she knows about his affair. The two argue, but reconcile and Cora announces that she is pregnant. She speculates that the new life they have created may balance out the one that they took. They go to the beach and swim, realizing that they still love each other. On the way back, Frank accidentally crashes the car and kills Cora.
Frank is tried and convicted for killing Cora. While on death row, he is visited by District Attorney Sackett, who confronts him with the evidence of his involvement in Nick’s murder and reasons that if he resists his legal fate in Cora’s death that he’ll only wind up back where he is with a conviction for Nick’s murder. Frank accepts that while he is innocent of Cora’s murder, his execution will be fitting punishment for his murder of Nick. Frank muses that just as the postman always rings a second time to make sure people receive their mail, fate has made sure that he and Cora have both finally paid the price for their crime.