In 1970, in the desert town of Harmony, New Mexico, a dim-witted criminal named “Weasel” robs a couple at gunpoint at the Harmony Motel. He steals their convertible and drives it to a ramshackle property owned by “Slue,” Weasel’s large and surly boss. Slue’s girl friend, Pearl, discovers a baby boy in the back seat of the stolen car. Slue reprimands Weasel for taking the child and threatens to feed it to the hogs. Pearl forbids Slue from hurting the baby and vows to leave him if he does. Soon after, a policeman shows up in search of the missing couple and their baby. Slue fires a cannon at the man, then takes the baby from Pearl’s arms and holds him up in victory. Six years later, Slue has named the child “Sonny Boy.” As a birthday “present,” he cuts out the boy’s tongue, rendering him mute. Sonny Boy is held captive in a silo, and taken out only to be tortured. Over the years, Pearl pretends to nurture him while allowing the sadistic abuse to continue. By the time he reaches seventeen, Slue determines Sonny Boy has come of age and tells his partners, Weasel and Charlie P., that it is time to put the feral adolescent to work. The mayor of Harmony has demanded a bigger take of Slue’s illegal earnings. Using an ice cream truck as a cover, Slue drives to the mayor’s house with Sonny Boy chained up in back. Slue lets the young man loose, and Sonny Boy crashes through a window and attacks the mayor. They tussle, but Sonny Boy quickly overpowers the mayor by dunking his head into a bathtub filled with water. Having taken a life for the first time, Sonny Boy catches himself in the bathroom mirror and furiously rinses off his face, wondering if he has become the pitiful thing he sees in the reflection. Later, Charlie P. and Weasel join Slue and Pearl for dinner. Pearl reminds the men to leave some leftovers, but Slue tells her he wants to keep him hungry. Later that night, Pearl sneaks food out to the silo while Slue informs his cohorts of plans to rob wealthy people in Los Angeles, California. They drive to the Los Angeles suburbs, where Slue instructs Sonny Boy to kill a priest. Sonny Boy ponders the murder after it is done, and decides he feels differently now that he has tasted the blood of a good man. Charlie P. suggests using Sonny Boy to rob a local prospector in Harmony who is rumored to be in possession of gold nuggets, but Slue rejects the idea of killing a local. However, Charlie P. convinces Weasel to help him “borrow” Sonny Boy. One day while Slue is away, they steal Sonny Boy out of the silo and take him to the prospector’s house. Although conflicted about obeying someone other than Slue, Sonny Boy easily kills the man and retrieves the gold. Weasel rewards him by playing with him like a dog, but Sonny Boy becomes overexcited and bites off Weasel’s thumb. Later, Weasel brags about the incident at a local bar. He is overheard by “Doc” Max Bender, an alcoholic physician who is rumored to have used monkey parts in human transplants, and Sandy, a tough local woman who tells Weasel to stop boasting. The next day, Slue comes into Sonny Boy’s silo and sees Weasel’s thumb hanging around his neck. He demands to know what happened, but Pearl distracts Slue and he follows her out, leaving the silo door open. Sonny Boy escapes and runs to an abandoned house, where he sees a young couple having sex. They catch Sonny Boy staring and attack him. Sonny Boy fights back, and the sheriff, who is in cahoots with Slue, arrives. The sheriff shoots Sonny Boy with a tranquilizer gun and takes him to Doc Bender’s house, where he instructs the doctor to stitch him up. The doctor is appalled when he discovers the young man’s tongue has been cut out. Later, as Slue and Pearl arrive to retrieve him, Sonny Boy escapes again. He hobbles through town, drawing an angry mob in his wake. When they corner him, he attacks. A frightened local runs into the bar and announces a “monster” is on the loose. Three days later, Sonny Boy has retreated to the desert with Rose, a sympathetic teenage girl who dreams of getting out of Harmony. As they kiss, Slue appears, hits Rose over the head, and drags Sonny Boy back to his car. That afternoon, Sandy incites the angry mob to set fire to Slue’s property. Slue fights back with cannons, but the mob advances closer to the house. Slue leads Pearl and Sonny Boy into a wooden pyramid where he stores stolen goods. Pearl is shot dead, and Slue cries out in horror. Later, Doc Bender and Rose observe the smoldering remains of the pyramid, believing that everyone inside is dead. Miraculously, Sonny Boy emerges from the debris. Doc Bender takes him back to his office, where he performs a tongue transplant. Although the operation is successful, he laments that Sonny Boy is still irreparably damaged. Sonny Boy fights to speak, and his first word is “no.” At night, he dreams about the fire and Slue embracing him. He wakes up to the sound of Rose’s motorbike and goes to the window. Looking at Rose, Sonny Boy wonders who he is, because even though he now has words, his pain has not stopped.