In 1931, a Missourian meat heiress is robbed by three men, who panic after murdering her boyfriend and kidnap her. At their hideout, the three are ambushed and killed by Eddie Hagan, who happened to witness the crime, and the rest of the notorious Grissom Gang.
Barbara Blandish is held captive by the gang, including Slim Grissom, a mentally handicapped thug who falls in love with her. Ma Grissom, the gang’s boss, sends a ransom note to the girl’s father, John P. Blandish, demanding a million dollars for her return. But she has no intention of returning Barbara, and the plan to kill her meets the disapproval of Ma’s husband Doc.
Private detective Dave Fenner is hired by Barbara’s father as weeks go by. After at first insulting Slim as a “halfwit” and repelling his advances, Barbara realizes that the only thing keeping her alive is his desire for her, Slim vowing to kill any gang member who harms her. She reluctantly becomes Slim’s lover.
Nightclub singer Anna Borg has no idea what became of her boyfriend, one of the kidnappers who got killed. She pulls a gun on Eddie, who lies that Anna’s boyfriend ran off with another woman. Anna allows herself to be seduced by Eddie, who then murders two men with knowledge of the crime.
Months go by. Fenner, out of ideas, poses as a theatrical agent who can help Anna’s singing career. He gets her talking about past criminal associations and learns where the missing girl might be. A furious Eddie kills Anna, then goes after Barbara only to have Slim stab him to death. Ma uses a machine gun to fight police and kills her husband Doc when he tries to surrender. Slim dies in a hail of bullets, but when Barbara weeps over him, her disgusted father walks away.