The Killer Inside Me opens in and around a diner. A homeless man is hanging around outside, so Deputy Lou Ford shows his badge and tells the bum that he must move away from the diner and ideally out of Central City. To emphasize the message, Lou puts out a cigar on the bum’s hand.
Generally perceived as a nice guy, though a bit dull, Lou is often given the task of running the unwanted out of town. Three months earlier, Sheriff Maples had told him to drive a new prostitute, Joyce Lakeland, out of Central City. When Lou warns her away, she becomes furious. She yells at Lou, slaps him, and then knees him in the chin. Her violent reaction releases “the sickness” again. He beats her senseless, almost to death—and she finds this sexually exciting. They become lovers.
When Lou speaks with Joe Rothman, a town labor leader, Joe asks Lou how he felt about his adopted brother, who had died several years ago, and about whom there had been a scandal. Lou goes home, where he finds his girlfriend Amy waiting for him, upset that they have not spent more time together recently. Amy pressures Lou to leave and then to marry her, claiming he will have to because she is pregnant. Lou counters that his father had him sterilized when he was younger.
Chester Conway, the richest man in the area, visits Lou to ask him to roust Joyce from Central City. Conway’s son, Elmer, has become too attached to Joyce, and Conway disapproves. Lou refuses to attempt bribing Joyce, but says he will help resolve the situation. When Elmer comes to talk to him, Lou suggests that Elmer leave the area with Joyce.
After Elmer agrees, Lou goes to Joyce and beats her into a coma. When Elmer shows up, Lou shoots him, planting the gun in Joyce’s hands so that it looks like they killed each other. Lou talks with the authorities and then heads home, stopping on the way to visit with Johnnie Pappas, the diner owner’s son, at the gas station.
Amy is waiting for him at his house. She is mad at Lou for getting mixed up in the killings, and madder still when she realizes that he has been with Joyce. She starts to walk out, but Lou wins her over by promising to marry her.
As soon as she leaves, the county attorney and sheriff show up. It is clear they think Lou may have killed Joyce, but he largely puts their suspicions to rest. They do tell him that Joyce is not dead and that Chester Conway is flying her to Fort Worth for medical treatment. Sheriff Maples and Lou go to Forth Worth as well, and Lou ends up sitting around while Maples gets drunk and mumbles about something Conway had said to him. When they get back to Central City, Joe Rothman quizzes Lou about the killings. Joyce has now been declared dead at the hospital.
Once back home, Lou stumbles across a picture of his old housekeeper, Helene. The sight of her scarred thighs brings back a memory of...