Based on a true story. Kate Barker (Shelley Winters) loves her four sons and would do anything for them. In an attempt to improve their lives, she leads them on a violent spree of bank robbery, murder, and kidnapping.

Raped by her brothers at age seven, Ma Barker exacts absolute loyalty from her four sons, sadistic Herman, narcotics addict Lloyd, soft-spoken Arthur, and homosexual Fred. During the Depression Ma and her brood leave passive Pa Barker and the Ozarks to begin a criminal career. While robbing a ferryboat eldest son Herman loses his temper and stomps a passenger to death. To console Herman, Ma sleeps with him. Apprehended robbing a picnic, Fred and Herman are incarcerated. In prison Fred succumbs to the advances of Kevin Dirkman, who becomes Ma's lover upon release. Over her protests Herman introduces his mistress, the prostitute Mona, into the band. After Lloyd rapes local resident Rembrandt, Ma, Herman, and Kevin drown the girl in the bathtub. Later Lloyd dies of a drug overdose. The gang then kidnaps congenial multi-millionaire Sam Adams Pendlebury. Upon payment of the ransom Ma insists that the hostage be killed. Her sons, however, release the multi-millionaire and Herman assumes command of the band. At their Lake Weir hideout Herman and Kevin unknowingly reveal their identities by firing machine guns at an alligator. Trapped by police, the Barkers fight to the finish, Ma being the last to fall.