In rural Missouri, during a sweltering summer in the early 1930s, nightclub photographer Heinie tips off small-time hoodlums Ed Bailey, Frankie Connor and Sam that heiress Barbara Blandish will be wearing an expensive diamond necklace at a roadhouse that night. Intending to steal the necklace, the thugs approach Barbara as she is about to drive off with her boyfriend, Jerry McGowan. When Jerry resists, the thieves shoot him and in a panic, kidnap Barbara. Stopping at a gas station, Frankie phones his girl friend, singer Anna Borg, to tell her they will be hiding at Johnny Hutchins’ shack in the country. By coincidence, fellow hoodlum Eddie Hagan, a member of the Grissom gang, stops at the station and sees the thugs drive away with Barbara. After hearing a radio broadcast about Barbara’s kidnapping, Eddie ascertains from the gas station attendant, who overheard Frankie’s conversation with Anna, that the thugs are headed for Johnny’s shack. Soon after, Eddie, accompanied by three members of his gang, Slim Grissom, Woppy and Mace, arrives at Johnny’s shack, where after killing Frankie, Bailey and Sam, they kidnap Barbara and pay Johnny to keep silent. At the squalid Grissom house, the sullen, spoiled Barbara tries to reason with Ma, the mustachioed matriarch of the gang, who decides to send a ransom notice to the heiress’ wealthy father, John P. Blandish, demanding $1,000,000 for her return. Later, when Ma’s husband Doc expresses misgivings about the kidnapping, Ma reassures him that only kidnappers who return their victims get caught, and she intends to “dispose” of Barbara. Upon receiving the ransom note, Blandish hires private detective Dave Fenner to act as a go-between. Meanwhile, Ma’s psychopathic, simpleton son Slim becomes smitten with Barbara, who calls him a “filthy, cretin half-wit,” sending him running out of the room, crying for Ma. After Ma retaliates by slugging Barbara and blackening her eye, Slim finds Barbara terrified, slumped across her bed. Slim kisses her, but when he realizes that he is impotent, he sprawls across her body and weeps. Once the ransom is paid, Ma proposes that they buy an interest in a Kansas City bar called Rocky’s Café and convert it into a “swanky” speakeasy. When Eddie, a slick-haired dandy, taunts love-struck Slim about getting rid of the “broad,” Slim declares that he will kill anyone who touches Barbara, including his own mother. Later, Ma and the others go to see Rocky to strike a deal about investing in his place. Also present is Anna, who performs there. Leering at Anna, Eddie follows her into her dressing room, where she points a gun at him and demands to know what happened to Frankie. After the wily Eddie lies that Frankie took Barbara and left town, Anna, thinking that Frankie double-crossed her, accepts Eddie’s advances. Realizing that the gas station attendant and Heinie can link him to the kidnapping, Eddie kills the attendant, and after forcing Heinie to place an anonymous phone tip to the police identifying Frankie and Bailey as the kidnappers, kills him, too. Later, when Slim brags to Barbara that he “saved” her from being killed by the gang, she realizes that the ransom has been paid and that her life depends on keeping Slim happy. In an act of self-preservation, she seduces him, and upon waking up the next morning, sobs in degradation. The next day, Slim goes to town and returns wearing an ill-fitting suit and bearing gifts for Barbara. Meanwhile, determined to find Barbara, Fenner discusses the case with police chief McClaine, and although McClaine is certain that Barbara is dead, he recalls the anonymous phone call linking Frankie to the crime and suggests that Fenner talk to Anna. Two months later, Slim escorts Barbara to their new "love nest," a garish, windowless, two-room apartment, complete with a gold leaf toilet and monogrammed towels. Soon after, Fenner, posing as a New York theatrical agent, visits the ambitious Anna and informs her that a “millionaire” client of his is backing a Broadway play and wants her to be its star. When Fenner warns that Anna’s past association with Frankie may damage her career and suggests that it could be counteracted if she could provide a clue to solve the kidnapping, Anna, blinded by dreams of stardom, reveals that Frankie told her he was going to Johnny’s place on the day of the kidnapping. Just then, Eddie walks in the door and recognizes Fenner. After Fenner punches Eddie and leaves, Anna tells Eddie that she told Fenner about Frankie’s phone call. Agitated, Eddie throws her on the floor, and when she pulls a gun, he shoots and kills her. Fenner then goes to see Johnny, and as they talk, the Grissom gang surrounds the house. Just as the gang hurls a grenade into the house, the police arrive and the gang quickly withdraws. Afterward, Slim discovers that Eddie has abducted Barbara and hurries to confront Eddie at his apartment. As Eddie assaults Barbara, Slim arrives, unsheathes his knife and maniacally stabs Eddie. Soon after, the police raid the speakeasy, and Ma, preparing for a fight to the death, readies her Tommygun. Doc, however, wants to surrender, and when he turns to leave, Ma guns him down. Arriving at Rocky’s in the midst of a gun battle between the gang and the police, Slim and Barbara speed away with police motorcycles in pursuit. Upon stopping for gas, Slim learns from the attendant that there were no survivors in the “downtown” shootout. Driving off, Slim wails for his dead mother. Later, when Blandish sees Slim and Barbara’s love nest, he declares with disgust, “She would be better off dead.” That night, after their car runs out of gas, Slim and Barbara take refuge in the barn, but the farmer spots them and notifies the police. In the barn, after Slim declares he does not want to live without Barbara, she says that no one has ever loved her like that and tenderly kisses him. The next morning, Slim awakens to find that the police have surrounded the barn. McClaine has invited the press to cover the story, and Fenner and Blandish are there, too. When McClaine orders Slim to surrender, Slim fires his gun, then steps out of the barn holding his weapon. After the police riddle him with bullets, Barbara tentatively comes to the door, kneels over Slim’s body and sobs. Sickened, Blandish orders her to get away from Slim, and although she asserts she was only trying to stay alive, he tells her that Fenner will look after her, then walks off.