Drifter Frank Chambers hitches a ride with local District Attorney, Kyle Sackett to a roadside diner and filling station called Twin Oaks. Twin Oaks’ owner, Nick Smith, is an older man with a beautiful, young wife named Cora. Nick hires Frank to work for him, but Frank really takes the job because he has the hots for Cora.
Cora does not love Nick, and she and Frank have an affair. The two eventually decide to run away together, but must leave on foot as they do not have a car of their own to use. Along the trek, Cora decides that if she divorces Nick, she will wind up penniless and back where she started; at the bottom. So, she returns home to the service station where they arrive in time to retrieve a goodbye note to Nick where Cora professes her love for Frank.
When Frank makes an offhand comment about killing Nick, that begins Coras wheels spinning. She soon convinces Frank to murder Nick so they can get the diner. Frank devises a plan to hit Nick over the head with a sock of ball bearings while he’s in the bath to make it look like he slipped and fell in the tub.
However, while Cora prepares to hit Nick in the bathroom, a police officer stops by where Frank is outside the diner keeping watch. While talking, they spot a cat climbing a ladder, and the cop says he likes cats and then leaves. In the meantime, the cat causes a power outage when it hits some wires on the ladder, and it’s killed instantly. This happens as Cora strikes Nick in the head. While horribly injured, Nick survives.
His survival is a blessing in disguise for the two lovebirds as the officer and DA Kyle Sackett suspect foul play, but Nick has no idea how he got that bump on his head. With Nick in the hospital for the next week, Frank and Cora spend the time in love and running Twin Oaks.
On the day of Nick’s return home, the same officer stops by, and tells Frank he saw Cora driving Nick home. Frank knows there’s no future for him there, so he packs his things and leaves to Los Angeles before they arrive.
A couple of weeks pass, and Frank still can’t get Cora out of his head. He begins hanging out as the marketplace Nick and Cora bought their produce in hopes of running into Cora again. One day, he spots Nick’s car, and then Nick himself. Nick convinces Frank to come back to Twin Oaks with him for some big news.
When the two return home together, Cora is surprised, but makes them all dinner. That’s when Nick announces he’s selling Twin Oaks, and he and Cora are moving to Canada to take care of his paralyzed sister there. Cora is clearly upset, and does not want to move.
Later that night, Frank finds Cora in the kitchen with a knife. He thinks she is going to kill Nick, but she says she was going to kill herself. She convinces Frank to kill Nick so the two can live happily ever after running Twin Oaks.
The next night, the three of them drive to Santa Barbara so Nick can sign the papers with Twin Oaks’ new owner, and to also introduce Frank to him so he can manage the place. Frank comes up with a scheme to make Nick’s death look like a drunk driving accident.
However, as they prepare to head out, Sackett arrives to fill a tire with air. Cora fakes an argument with Nick to say that he and Frank are too drunk to drive, and she gets behind the wheel. That way, Sackett can be a witness that Nick and Frank were drunk ahead of the accident.
Cora drives them to a dangerous cliff road, and stops the car because it’s overheating. Frank hits Nick over his head with a bottle; killing him. They start the car, and let it go over the cliff’s edge with Nick in it. Then the plan is to make themselves look injured below as if they were all in the car together when it went over the side.
However, the car doesn’t fall far enough, so Frank gets inside the car to get it to go further down. It does, but Frank is still inside it when it begins rolling; injuring him too. Sackett has been following the trio since they left Twin Oaks, and he arrives to find Cora screaming for help.
Sackett believes that Cora and Frank killed Nick, but does not have any proof. At the hospital, Sackett tries to get the two to turn on each other when he convinces Frank to sign a complaint that Cora plotted Nick’s murder. Frank agrees, and Sackett brings Cora up on murder charges.
Sackett’s plan is short lived though when Cora’s attorney, Arthur Keats, shows up. Keats knows Sackett’s game, and he gets Cora to give a full confession to one of his men (unbeknownst to her at the time) instead of the D.A. which would hurt her chances for winning the case.
Since Sackett’s plan fails, he doesn’t have the evidence he needs to win his case, and Keats gets him to reduce the charges to manslaughter which Cora pleads guilty to. The court gives her probation for Nick’s death.
Publicity from the trial generates a lot of business for Twin Oaks in the weeks after the incident. Frank still works for Cora, but their relationship is strained. One day Keats and Sackett show up for some lunch. Keats pulls the two aside, and lets them know that Sackett might still take action against them based on people’s gossip of an unmarried man and woman living under the same roof in sin.
To give their relationship an air of legitimacy, Cora and Frank wed, but at the reception, Cora learns her mother is sick, and she must go tend to her. At the train station, Cora and Frank get into a little spat, and then she leaves in a huff. Frank spots a woman with car trouble, and helps her out. The two then take a little trip to Mexico for a quick fling.
Eventually, when Cora returns, she has a much better attitude towards their relationship. However, this is interrupted when Kennedy, the man who worked for Keats (and transcribed Cora’s murder confession) shows up. He and Keats had a spat of their own, and Kennedy took Cora’s signed confession on his way out of Keat’s office. He blackmails Frank and Cora for $15,000 to keep quiet.
Frank beats Kennedy up, and forces him and his partner to turn over all the evidence they have. While this is going on, Frank’s Mexico fling shows up, and tells Cora all about their romp while she was gone. Angry, Cora threatens to tell Sackett about Frank’s part in Nick’s murder, and then tells him she’s pregnant with Frank’s child.
Cora believes this baby will make up for the life they took, and wants to make amends. That night, the two go to the beach, and take a swim. They swim so far out that Cora is too weak to swim back on her own, but she only swam that far out to prove to Frank she trusts him. He proves that he trusts her when he helps her get safely back to shore.
Now that their trust issues are all cleared up, the two agree to dedicate themselves to raising their baby. On the drive back though, Frank takes his eyes off the road, and crashes the car; killing Cora and her baby.
Sackett arrests Frank. He’s tried and convicted of killing Cora (which ironically though, he did not). On death row, Sackett visits Frank one last time, and tells him that even if he were to get a retrial and beat the murder charges for Cora, he now has enough evidence of his involvement in Nick’s murder. So it would only be a matter of time before he winds up back where he is, but this time with a conviction for Nick’s murder.
Frank accepts his fate, and asks the priest to pray for him and Cora. He says 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 pay for their crime a second time.