In the oppressive heat of a Texas trailer park, Chris Smith, a desperate young man with a debt over his head, cooks up a plan to have his estranged mother killed for her $50,000 life insurance policy. Reeling from his own poor choices and pressured by debt collectors, he enlists the help of his father, Ansel, and stepmother, Sharla, to hire a hitman. In the midst of family dysfunction and economic despair, Chris's plan becomes the only perceived ticket to a better life. The family agrees to the scheme, grasping at the promise of financial salvation.

Enter Killer Joe Cooper, a Dallas police detective with a chilling sideline as a contract killer. When Joe meets with the Smiths to discuss the job, he exudes a stone-cold professionalism that veils his underlying menace. The cash-strapped family cannot pay Joe's upfront fee, leading him to suggest an unorthodox collateral: Chris's innocent young sister, Dottie, whose ethereal innocence contrasts with the pervasive moral rot around her. As Joe integrates himself into the family's life, under the guise of a protective engagement to Dottie, the lines between the professional and the personal start to blur. The agreement spirals into a tight web of deceit, manipulation, and violence, laying bare the grim realities of the characters' lives and pushing them towards the brink of catastrophe.