Young Kate Barker is brutalized by her father and older brothers, who rape her. Thirty-five years later, the middle-aged Kate 'Ma' Barker, now brutalizes innocent people herself, while indulging her monstrous sexual appetites. She lives by robbing banks with her four sons; the pragmatic Arthur, the sadistic Herman, the bisexual Fred, and the loyal, drug-addicted Lloyd. It all begins in the late 1920s when Ma leaves her husband, George, and her Arkansas home and embarks on her own with her four sons on a robbery-murder spree to make her own fortune, while keeping them under a tight leash.
When Herman and Fred are arrested and imprisoned for petty theft charges, Ma takes over the group and leads Arthur and Lloyd on a bank robbery spree to gain enough money to get her sons out of jail. The gang is joined by a gunman, Kevin Dirkman, who was Fred's cellmate during his incarceration (and his strongly implied lover). The group is also joined by a local prostitute, Mona Gibson, whom Herman frequented before his imprisonment. The gang resorts to more violent action and robberies.
While hiding out at a cabin in Kentucky, Lloyd is approached playfully by a young woman named Rembrandt, who swims up to him as he dangles his feet in the lake. The encounter begins flirtatiously, but quickly turns into rape and abduction, after Lloyd shows her the needle marks on his arm. Lloyd tells her he's a Barker, in spite of Ma's warning to use an alias. Not wanting the woman to report their location to the police, the Barkers hold her captive and Ma kills her by drowning her in the bathtub, despite the protests of her sons.
Some time later, the gang arrives in Tennessee where they abduct a wealthy businessman, Sam Pendlebury. Holding him for a $300,000 ransom, the sons, particularly Herman, bond with their captive whom they see as the decent father figure they never had. When Herman and Mona go to collect the ransom, they are chased by a pair of FBI agents and barely escape. The ransom is eventually paid in full, and the plan is to leave Sam untied at the hideout, giving them plenty of time to escape before he can talk to the police. But Herman wants to see Sam's eyes--which when uncovered, remind him of their father. Sam says he can't see any of them (almost certainly true, given his head trauma, and the fact he's always been straight with them, even when it put his life in danger).
Ma still insists Sam be killed, so as to avoid any possibility of his identifying them. He's led into the woods to be shot, but the boys, now seeing Herman as their leader, set him free, lying to Ma about killing him. (There is no indication this leads to the gang's ultimate downfall). Later, to explain why they need to leave the territory immediately, Herman tells Ma of the deception, and knocks her to the ground, saying she's no longer the boss. The trust between them is gone.
The gang hides out in Florida Everglades where Lloyd soon dies from a morphine overdose and Mona leaves Herman and the gang after she reveals that she's pregnant and does not want to be around them anymore out of fear for the safety of her unborn child, which Herman fathered. Her fears are justified when Herman and Kevin give away their hiding place a little later. A local handyman and caretaker, Moses, witnesses them shooting an alligator out on a lake with a Tommy gun and calls the police to report his suspicions. When asked, he says their cars have Tennessee plates, and the authorities quickly deduce these are the Barkers.
At the climax, a large contingent of FBI agents and local police arrive at the Barkers' hideout and a huge shootout ensues between the authorities and the surviving members of the gang. Kevin, Fred, and Arthur are all killed (along with many officers). Herman commits suicide to prevent himself from being sent to prison again. Ma is the last one to fall, firing her Thompson machine gun at the police, screaming in rage and anguish, unable to accept that her boys are dead because of her.
