A young college reporter prepares to interview a man in a mental ward. Instead of Girl, Interrupted this is Girl, Interrupting. But the man she's interviewing, Karl Childers, doesn't mind his busy day of sitting and staring out a window being interrupted. He eagerly tells the girl his whole life story.
Here it is.
Karl had a rough childhood. His parents kept him in a shed, where he slept in a hole in the ground. One day, he saw his mom engaging in, um, intimate relations with someone who wasn't his father. With a Kaiser blade—some people call it a sling blade, but he calls it a Kaiser blade—Karl killed the man having sex with his mother. That made his mom mad, and Karl realized that she was liking getting it on with that guy. So he killed her, too.
Okay, back to the present. The murder was a long time ago, and now Karl is set to get out of the mental hospital. The reporter asks Karl if he'd kill again, and Karl says, "I don't reckon I got no reason to kill nobody." The foreshadowing here is so strong, we imagine it won't be too long before he finds a reason to kill somebody.
Karl is released back into his small Arkansas hometown. He has nowhere to go, and he has a hard time adjusting to the freedom of the outside world. At a fast-food stand, he learns that he loves French-fried potaters, but he doesn't like much else about the world. He attempts to return to the mental hospital, but Mr. Woolridge, the administrator, says he can't just come back. It's a mental hospital, not a Motel 6. They haven't left the lights on for him.
Mr. Woolridge lets Karl spend one night in his own home, and then he gets Karl a job in town with Bill Cox, a man who repairs machinery. Karl has never met a lawnmower he can't repair, so it's a good fit. Cox even lets Karl sleep in the back room. Only a day after release, Karl has a job, money, shelter, and French-fried potaters. What more could a man want?
Well, Karl wants a friend. He meets a young boy named Frank at the laundromat and helps him carry his "warsh" home. Frank likes the funny-sounding older man, even after Karl tells Frank that he was in a mental hospital for killing someone. Frank introduces Karl to his mother, Linda, and Linda offers to let Karl sleep in their garage. Karl moves in.
For as nice as Linda is, her boyfriend, Doyle, is the opposite. He is mean to Frank, says homophobic things about Linda's friend Vaughan, and is cruel to Karl. Frank not only wishes his mom would stop seeing Doyle, but he also wishes that his own father, who killed himself, were alive again.
This isn't Pet Sematery, so that's not going to happen. Instead, the family tolerates Doyle's verbal abuse and threats of physical violence. He says he'll kill Linda if she tries to leave him. One night, he invites all his friends over to play music, but since he's a man incapable of having friends, the night ends with him yelling, screaming, and physically throwing the guys out of Linda's house.
Linda has had enough, so she kicks Doyle out, too. Good riddance.
Over the next few days, Karl gets baptized and he visits his own father, who is still alive and swears he never had a son. Karl tells his demented dad that he used to think about killing him, but now he sees that the geezer will be dead soon, so the world will be rid of him all on its own. Good riddance, part two.
At Linda's, the peace doesn't last. Doyle soon comes groveling back, and Linda accepts him. Doyle wastes no time saying cruel things to Frank and telling Linda that he wants Karl out of the house. Karl goes to Linda's friend, Vaughan, and gives him all the money he's made fixing lawnmowers. He wants Vaughan to keep Linda out of the house for the night—and to give her the sack of money.
That night, Karl returns to Linda's house, where Doyle is by himself. With a blade, he kills Doyle, then calls the police and turns himself in.
We last see Karl back in the mental institution, which might be his one true home. When asked how the outside world was, Karl says it was "too big." Inside the ward, our murderous little Goldilocks Karl finally feels just right.