Lou Ford is the amiable, soft-spoken deputy sheriff of a small, dusty Texas town. Beneath his folksy charm and helpful demeanor, however, lies a deeply disturbed sociopath carrying a terrifying childhood trauma and a dormant, violent urge he calls "the sickness."The plot kicks into gear when Sheriff Howard sends Lou to run a small-time prostitute named Joyce Lakeland out of town. Instead of merely evicting her, Lou initiates a brutal, sadomasochistic affair with Joyce. Rather than repelling him, her fierce independence and willingness to embrace his dark fantasies fuel his obsession.Complications multiply when wealthy local construction magnate Elmer Conway, whose son Elmer Jr. is Joyce’s primary client, discovers their liaison. Elmer hires the corrupt union boss Chester Conway to eliminate Joyce and pay off Lou to look the other way. Recognizing an opportunity to frame Elmer Jr. and eliminate multiple problems at once, Lou orchestrates a horrific double-murder scheme. He beats both Joyce and Elmer Jr. nearly to death in her home, staging the gruesome scene to look like a lovers' quarrel gone fatally wrong, and disposes of their bodies.Miraculously, Joyce survives the vicious assault in the morgue, and
district attorney Howard Hendricks manages to revive her. Though traumatized, Joyce is terrified of Lou and initially keeps his identity secret, agreeing to help frame Elmer Jr. instead. Meanwhile, Lou grows increasingly paranoid. Pressured by the lingering threat of exposure and the relentless probing of DA Hendricks, Lou’s grip on reality frays entirely.To silence the loose ends permanently, Lou murders Joyce in her hospital bed, framing it as a suicide. He then sets his sights on anyone growing suspicious of him. He brutally kills Amy Stanton, his sweet, unsuspecting schoolteacher fiancée who has figured out the truth about his dark nature. Lou also frames Chester Conway, leading to a fiery showdown where Chester dies in a suspicious explosion.In the climax, Hendricks and the state police finally corner Lou at his isolated rural cabin, having uncovered the overwhelming
forensic evidence linking him to the murders of Joyce, Elmer Jr., and Amy. Rather than surrender to the law and face the electric chair, Lou sets fire to the house with himself inside. He chooses to perish in the flames, ultimately consumed by the very inner darkness that defined his tortured existence.