Billy Halleck is an overweight attorney living in Fairview, a wealthy Connecticut suburb, where he has recently been made partner at his law firm. He lives with his wife, Heidi, and their 14-year-old daughter, Linda. At 246 pounds, Billy steps on his bathroom scale and recalls an unsettling encounter outside the local courthouse. After a vehicular manslaughter hearing in which Billy was cleared of killing an elderly Gypsy woman named Susanna Lemke, her ancient father, Taduz Lemke, a man with a horribly rotting nose, touched Billy's cheek and whispered a single word: "Thinner." The hearing was presided over by Judge Cary Rossington, a golf buddy of Billy's who did not recuse himself. Police Chief Duncan Hopley helped bury the investigation. No one in Fairview objected because the victim was a Gypsy.
Billy's comfortable life continues. He wins a major lawsuit, and he and Heidi vacation at Mohonk, a resort in upstate New York. During the trip, Heidi notices Billy has lost weight despite eating as much as ever. A penny scale reads 232, a 14-pound drop. Billy is unsettled not by the number but by the fortune printed below it: the single word "THINNER." On the drive home, Heidi urges him to see their doctor, worried the weight loss could signal cancer.
Over the following weeks, Billy's weight continues to fall despite deliberate bingeing. Dr. Michael Houston, the family's country-club physician, runs extensive blood work and finds nothing wrong. He speculates Billy may be "thinking himself thin." Billy is relieved, but the relief is short-lived: His pants nearly fall down in court, and examining himself in a bathroom mirror, he sees how dramatically his body has changed. He believes fully that the Gypsy cursed him. That night the scale reads 195.
Billy confesses to Heidi that the weight loss is continuing but withholds his theory about the curse. A flash of intense hatred seizes him as he recalls the truth of the accident: Heidi had been sexually distracting him while he was driving, preventing him from braking in time when Susanna Lemke stepped into the street. The accident was significantly Heidi's fault, yet the old Gypsy cursed Billy, not her. This knowledge breeds a corrosive resentment that deepens throughout the novel.
Desperate for answers, Billy visits Leda Rossington, the judge's wife, who is haggard and terrified. She reveals that after the old Gypsy touched Rossington, his skin began transforming into thick, reptilian scales. His hands have become claws, and the scaling has invaded his mouth. Rossington has gone to the Mayo Clinic. Leda blames Billy and throws a martini glass at him as he leaves.
Billy then visits Hopley, who is barricaded in his darkened house. Hopley briefly illuminates his face: It is a horrifying landscape of enormous pimples and oozing whiteheads. The old Gypsy tracked Hopley to a restaurant and touched his face. Hopley delivers a bleak speech about the Gypsy's lifetime of persecution, arguing that a man seeking revenge does not want to hear about shared blame. He suggests Billy consider killing the old man.
Billy checks into the Glassman Clinic for a metabolic series. The doctors are baffled but cannot diagnose him. Meanwhile, Hopley shoots himself with his service pistol. Dr. Houston and Heidi, now convinced Billy has a mental health condition, obtain a committal order to involuntarily place him in a psychiatric facility. Linda is sent to stay with her Aunt Rhoda. Billy is furious and contacts Richard Ginelli, a charismatic New York organized-crime figure he befriended through legal work, asking for help.
Billy's law partner, Kirk Penschley, engages investigators to trace the Gypsies, who have been moving north along Interstate 95. Taduz Lemke is listed as 106 years old. Susanna was his daughter, not his wife as Billy had assumed, a revelation that deepens Billy's understanding of the old man's grief. At 156 pounds, Billy writes Heidi a letter explaining his plan to confront Lemke and leaves home.
He follows the Gypsy trail up the Maine coast, showing photographs and passing out cash for information. His skeletal appearance shocks strangers. He locates the Gypsies camped near Bar Harbor and walks into their camp. Gina Lemke, Taduz's great-granddaughter, spits on him. The old man emerges, dismisses Billy as "ignorant scum," and refuses to lift the curse. Billy argues that blame is shared. Gina shoots him through the left hand with a ball bearing from her slingshot, and he is thrown from the camp.
Badly injured, Billy calls Ginelli, who arrives in person, listens to the full story, and believes it without hesitation. Over three nights, Ginelli wages an escalating terror campaign against the Gypsy camp: He poisons their fighting dogs, fires an assault rifle over their heads, and lures Gina away to deliver an ultimatum. Either Lemke removes the curse, or Ginelli will harm the Gypsy children. The campaign works. Gina calls to report that Lemke will meet Billy at a park in Bangor.
Lemke appears on a park bench beside Billy, holding a strawberry pie in a disposable aluminum plate, its crust pulsing faintly like something alive. He explains that the curse is a living thing, not a static spell. He instructs Billy to bleed into the pie through his wounded hand, transferring the curse into it. To be fully rid of the curse, Billy must get someone else to eat the pie, or it will return double. Billy bleeds into the pie while Lemke chants. The wound heals instantly. The slit in the crust seals itself. Lemke departs with a warning: "Be careful who eats the meal that was meant for you."
Billy returns to the car where Ginelli had been waiting and finds only Ginelli's severed hand on the seat, filled with ball bearings. The Gypsies have killed his friend. In shock that hardens into cold resolve, Billy drives south and calls Heidi, demanding she rescind the committal order. She agrees. He tells her he is bringing a strawberry pie, her favorite.
Back in Fairview, Billy presents the warm, faintly pulsing pie to a grateful Heidi. He goes upstairs and lies in the dark, listening to the sounds of her cutting a piece and eating. He falls asleep with grim satisfaction.
The next morning, Billy discovers that Linda has come home unexpectedly from Aunt Rhoda's, carrying a Greyhound-tagged suitcase and roses as a peace offering. In the kitchen, two plates and two forks sit in the sink. Both Heidi and Linda have eaten the pie. Billy stares at what remains, its crust still pulsing. Then he gets himself a plate and a fork. "Eat hearty," he whispers, and cuts himself a piece.