Frank is unwillingly drawn into the web of crime to free Cora from her imprisoning marriage - he prepares a small burlap of ball bearings:

I-I guess I worked out the details, but the original plan was hers. She got it from an article in a magazine that said that the most serious accidents happen right in people's homes, mostly right in their own bathtubs.

They connive and scheme together to get their stories straight and plan the execution of the murder:

Frank: We're gonna sink or swim in how we tell that story.
Cora: Well I won't miss. Nick was taking a bath. You were outside wiping off the car. I was ironing in the kitchen. All of a sudden, I noticed some water dripping from the ceiling.
Frank: Maybe we'd better say that...
Cora: Don't change a word of it! We've got it all set and I know it backwards.
Frank: All right, all right. But make sure he's in the tub when you go in, and just say you came in to get some clean towels. And then, when he's not lookin', ya...Then, whaddya do?
Cora: Then I lock the door and make sure the water's still running. I step out the window, down the stepladder.
Frank: Make sure you put the stepladder back in the shed. If anybody sees that stepladder, we're sunk.
Cora: And don't you dare move one inch away from that car in case anyone comes along.
Frank: Don't worry. If I give you the signal on the horn, you call everything off pronto.
Cora: But nobody ever stops here when all the lights are out in front.
Frank: Cora. Maybe it would be better if I did it.
Cora: We've settled that a dozen times. If I go in there, he won't pay any attention to me.

Tension builds - nervously, they both look up when they hear the sound of running bath water and Nick singing. Their plan goes awry when a motorcycle cop pulls into the driveway and happens to notice a stray cat climbing up the stepladder. The cop comments: "I like cats. Always up to something." [An unintentional double entendre that describes Cora.] In a sheer moment of terror, the cat electrocutes himself on the roof of the cafe and there is a grisly explosion. [The sexual, electrical chemistry between the two lovers results in an uncontrollable, unpredictable explosion - a overpowering metaphor of death and destruction.] The electrical circuits are shorted out, the lights turn dark, and Cora screams.

Due to quirky fate, their clumsy attempt at murder fails - Nick is unconscious but still breathing on the floor of the bathroom. To her disbelief, Cora is told that Nick must be revived: "There was a state cop out there and he saw that stepladder...If he dies, we're sunk and the cop saw that stepladder. If he dies, they'll know. If he dies, they'll get us." Cora frantically calls for a doctor, and Nick is brought to Blair General Hospital for treatment.

Outside Nick's hospital door, Kyle Sackett (Leon Ames), the neighboring district attorney, questions Cora about how Nick's accident occurred. She explains about "the big flash of fire and a terrible noise" and how the lights went out when Nick tumbled to the floor from the bathtub. Nick's physical condition is considered critical. He struggles to communicate and manages to relieve some of Cora's worry about her deliberate scheme to kill him when he can't remember anything: "Everything went dark. What happened?"

The two are followed on their drive home by the D.A. and the motorcycle cop. Suspicious of the potentiality of a deliberate murder, the perceptive Sackett asks about the misplaced stepladder and the fuse box - but as they view the "deader than a doornail" cat which was electrocuted when it walked on the bare wire and "the fuses blew out like a cannon," and Frank and the cop conclude that "Cats are poor dumb things...They don't know anything about electricity," the D.A. leaves assuming that "accidents can happen in the weirdest sort of ways."

Both Cora and Frank are guilt-ridden - Frank resolves that they won't make any more mistakes and there won't be a next time:

Cora: It was all my fault.
Frank: Mine too.
Cora: No, no. It was all my fault. I was the one that thought it up and you didn't want to, but next time, I'll listen to you.
Frank: Except there won't be any next time.
Cora: Never, never.

Although they are both scared and uncertain about their next move, Cora assures Frank, the one with "the brains," that he will be in charge from then on, but they first have to see whether Nick will survive:

Cora: But from now on, you'll be the brains of this outfit and I'll work. I'll work so hard for this place, Frank.
Frank: We can't make any plans till we find out about Nick.
Cora: Yeah, I know.

Cora is phoned and told that Nick "is going to be all right," but will remain hospitalized for a week. In one of the film's most frequently quoted lines, Frank links sex and violence:

Frank: Now we can just breathe again for a minute.
Cora: Just think. A week. A whole week to work things out.
Frank: Will you give me a big kiss before I sock ya?

Cora and Nick enjoy an idyllic week together. They frolic at night in the surreal surf and enjoy romantic trysting with the breathing room given them by Nick's absence:

It was the happiest I'd ever spent in my life. I wouldn't let myself think. And Cora wouldn't even discuss what was going to happen when Nick came home. All I cared about was her being happy. And as for me, I felt as if I was riding on a cloud.

However, when Nick is being driven home, Frank has only one option. He hurriedly packs and leaves without saying goodbye, and becomes a vagabond once more:

It was for her sake as well as mine that I knew I had to move on. If I'd waited for them to get back, I wouldn't have any arguments strong enough to Nick or give up the place and go away with me. And if I stayed there, I could see where she and I were headed. After a couple of weeks in L.A., I-I sunk low enough to hang around the wholesale market where they bought a lot of their stuff, hoping I, I'd run into her. I just couldn't get her out of my mind. I couldn't get her out of my mind. It kept naggin' me all the time.

Obsessed and drawn back by the memory of angel-faced Cora, Frank locates Nick's car at the Los Angeles market, and with only a half-hearted protest, he is convinced to return with Nick to Twin Oaks. In the cafe, Cora is stunned to see Frank re-appear: "Well, where'd you come from?" In the kitchen, Frank is curious about whether she has missed him - she averts her gaze and represses her sexuality. She tries to occupy herself with stirring soup or serving salad, while projecting a coldness in her stance:

Frank: Have you been thinkin' about me, Cora?
Cora: I couldn't forget ya that quick.
Frank: How have ya been?
Cora: All right.
Frank: Have you got a little kiss for me?
Cora: We're going to have dinner in a few minutes and you'd better get ready.
Frank: As a homecoming, this is the worst flop I ever saw in my life.

To their stunned and speechless surprise, Nick suddenly announces that he won't have any more exorbitant electricity bills because he is selling the cafe, retiring and moving to Northern Canada to live with his paralyzed sister where Cora will act as a nurse. Without conferring with Cora as his marital partner, he affirms that his "mind is made up":

I'm selling the Twin Oaks...It's not a joke...They're gonna turn this corridor into a four-lane highway. He's offering a big price for the place....But mainly Cora, it's so you can stop work and take it easy...We're going back to live with my sister...She hasn't been very well for a good many years...She's going to live for a long time, yet. I hope. But she needs us to take care of her, (To Cora) especially you, a woman.

Nick believes both Frank and Cora will benefit from his unilateral decision:

(To Frank:) In fact, my boy, you're gonna have a first class job with a future. And I predict as you get along in the world, you're gonna lose that itchy in the feet.
(To Cora:) Cora, my dear, in years to come, you'll thank me for this.

[Frank would presumably be made manager of the Twin Oaks, similar to what Cora was ultimately offering Frank -- if they could do away with her husband. In either case, Cora could "stop work and take it easy" -- with Frank making decisions and receiving sexual favors from Cora.] That night, Frank discovers Cora, wearing a black robe, in the kitchen with a long butcher knife in her hands. A stifled housewife, she is contemplating suicide and in despair over Frank's return:

Cora: Why did you come back?
Frank: I had to, that's all.
Cora: No, you didn't. I could have gone through everything if you hadn't come back. Why couldn't you just leave me alone and not come back?
Frank: Cora, let's, let's figure somethin' out. I love ya, Cora.
Cora: You love me? And whaddya do? You let him take me away to some miserable little dump of a town where I'll rot for the rest of my life while waiting on him and his half-dead sister. You love me. I love you but whaddya do? You let him take me to Santa Barbara and you're even gonna ride along in the car with us. You're gonna stay at the same hotel with us. Why, if you really loved me, you could...
Frank: All right.
Cora: No!
Frank: Yeah. I can't leave you.
Cora: But isn't there any other way out for us?

Frank concludes that they've "tried every other way" and it is now his turn to murder Cora's husband: "I guess it's in the cards":

That other time, it cured me of any idea we could pull a perfect murder. This was gonna be such a bad murder it wouldn't be a murder. A regular drunk automobile accident with liquor in the car and all the rest of it.

In their second attempt to kill the genial, fifty-ish husband, they get him drunk and arrange to drive him along the coast road to Santa Barbara for an appointment to sell his cafe. Sackett happens to pull into the gas station to fill his tires as they are leaving. He witnesses that both Nick and Frank are drunk - forcing Cora to drive, with Nick in the passenger's seat and Frank in the back seat. On the turnoff road to Malibu Lake ("the worst piece of road in Los Angeles County"), they pull over and Nick practices hearing his echo re-bounding back from the cliffs. Frank bludgeons a wine bottle over his head, silencing the voice (but not the haunting echo which comes back), killing him. Frank pushes the car off the cliff (with Nick in it) and asks Cora if she will be strong enough for their future ordeal:

Frank: It's gonna be tough going now. Are you sure you can go through with it?
Cora: After seeing that, I can go through anything.

They need to scale the cliff down to the car: "Come on, let's get down there. We gotta mess ourselves up so we can prove we've been in the accident too." Their less-than-perfect plan falls apart - the car is perched near the top of the cliff, and another car approaches on the road. Cora climbs back up to the road to yell for help and to get the attention of the car, while Frank attempts to push the car further down the rest of the way. As he gets into the car, the ground breaks loose and he is unable to get out of the car in time before it cascades over and over, rolling further down into the gulley. Cora's screams for assistance are real - and they echo back at her as she clutches her white handbag and reaches the top.

Their car was followed by Sackett, certain that they have just committed murder: "You can stop yelling, Mrs. Smith. Sure, I've been following you. It's too bad I couldn't have been closer behind." Frank is brought by ambulance (with Nick's corpse) to a hospital.

The sly district attorney confronts Frank in his hospital room and immediately wants to incriminate him for Nick's death:

Sackett: You and that girl murdered her husband, and the sooner you admit it, the better it'll be for you.
Frank: You're wrong, Mr. Sackett.
Sackett: How about a full confession? A quick plea of guilty and I'll do what I can for you with the courts. Clemency for you.
Frank: You're crazy.
Sackett: Would it interest you to know that I've been wise to you ever since that bathtub business?
Frank: Nick's death was an accident. Why should I want to hurt a nice, harmless guy like that that I was workin' for?
Sackett: A motive? Well, the girl herself, for one thing and a nice, paying business for another.
Frank: That's no good, Mr. Sackett. I never want to be tied down to anything or anybody in my life.
Sackett: All right. Then we'll come to the real motive. That brand new ten-thousand dollar insurance policy Nick took out on his life.

Frank vehemently denies knowing anything about the policy that Nick took out the day before he returned to the cafe. To neatly wrap up the case, Sackett wants to pin the murder on Frank - but then on second thought, he shrewdly proposes that Cora is the one responsible for the murder, because Frank was drunk and didn't "know what happened" [throughout the entire plot, Frank doesn't know what is going on, either drunk or sober!]:

Sackett: It all started when you and Cora Smith had a great idea. Nick's had an accident. Get him to take out an accident policy.
Frank: But I left that place before Nick came home from the hospital.
Sackett: And two days after you came back, he got killed. Oh, you were in touch with her by phone. And the day after the policy was granted, you accidentally, on purpose, ran into Nick and whaddya think? She'd already fixed up this little Santa Barbara trip, and of course, just for ol' times sake, you had to go along. Then, she had to see Malibu Lake. Wasn't that an idea, now? Would you like to pick it up from there?
Frank: No.
Sackett: No?
Frank: No.
Sackett: But it was all planned. You crowned him from behind. Then she slid out on the running board, held the wheel, and started the car. Then it was your turn to climb out so you could both claim you just escaped in time. But she moved too quick for you. You couldn't quite make it, so she jumped off and you were caught and went over the cliff.
Frank: NO! That's isn't at all what happened.
Sackett: How do you know what happened? You were drunk.
Frank: I mean, I mean, well, I-I don't think that's the way it happened.
Sackett: You were drunk. You even thought you were driving. You don't know what happened...Wait a minute. Maybe you didn't have anything to do with it. Maybe she did it. Listen, laddie, she did do it. There were just three people in that car - Nick and you and Cora. It's a cinch Nick didn't have anything to do with it. So if you were too drunk to do it, that leaves her.
Frank: Who-who-who says anybody did it?
Sackett: I do. Now if you told the truth at the inquest that you didn't have any interest in this girl except as the wife of your boss, then you gotta do something.
Frank: Do something? I-I-I don't follow you, Mr...
Sackett: You've got to sign a complaint against her.
Frank: A complaint?
Sackett: If you were in that car, drunk and helpless when she sent her husband over that cliff, then she tried to kill you too. You've got to do something about that, laddie, because somebody's gonna think it's pretty funny if you didn't.
Frank: She couldn't have meant to kill me.
Sackett: Now you were drunk. You couldn't know what was going on, could you?
Frank: I guess not.

According to Sackett, Cora's motivation for killing both of them was that "she wanted that sweet little piece of property and that ten thousand dollar insurance money all to herself instead of sharing it with you." The manipulative Sackett goads Frank into signing a complaint against Cora, destroying any remnant of trust they have for each other. After implicating her in the crime, Frank is assured that he won't be charged as Cora's accomplice in the murder trial:

It's you or her. If you didn't have anything to do with killing Nick Smith, you'd better sign this because if you don't, I'll know and so will the judge. And so will the jury. And so will that guy that gives you the business in the poison gas chamber in San Quentin. And so will the boys who bury you out there alongside all the others who were too dumb to make a deal while they still had a chance to save their necks.