Billy Halleck, portrayed by Robert John Burke, is a self-absorbed and excessively overweight lawyer residing in the quaint town of Castle Rock, Maine. He shares his home with his 14-year-old daughter, Linda, played by Bethany Joy Lenz, and his wife, Heidi, portrayed by Lucinda Jenney. Following a controversial legal victory defending an underworld crime boss, Richie "The Hammer" Ginelli, played by Joe Mantegna, Billy’s life takes a dark turn during a local carnival hosted by a group of Gypsies.
One fateful evening, Billy accidentally strikes an elderly Gypsy woman with his car due to a distraction from his wife. After a court hearing where he faces no consequences, the woman’s father, a centenarian warlock named Tadzu Lempke, played by Michael Constantine, places a devastating curse on Billy while uttering the chilling word "Thinner." This curse causes Billy to shed pounds at an alarming rate, completely irrespective of his eating habits.
As the curse wreaks havoc, the warlock also afflicts corrupt Judge Cary Rossington, portrayed by John Horton, and Police Chief Duncan Hopely, played by Daniel von Vargan. Both men confront gruesome fates—Judge Rossington metamorphoses into a lizard before taking his own life, while Duncan succumbs to grotesque mutations, leading him to commit suicide as well.
In denial about the supernatural ordeal unfolding before him, Billy remains skeptical until a visit to a clinic reveals the stark truth of his unexplainable weight loss. Recognizing the curse as genuine, he must seek out Lempke to have the curse lifted. As Billy’s family dismisses his claims and urges him to seek help, he distances himself, determined to reclaim his life.
His search takes him to a Gypsy troupe at a carnival in Connecticut, but his encounter with Lempke is futile. Facing danger, Billy is shot in the hand with a slingshot by Lempke's great-granddaughter, Gina. During his family's escalating concern for his safety, he enlists the help of Richie, who believes in the curse and vows to assist him.
In a despicable act, Richie poisons dogs belonging to the Gypsies and threatens them indirectly, only to face retaliation. Richie escalates the situation further by launching an aggressive attack against the Gypsy camp, resulting in chaos and bloodshed, including Gina's husband losing his life amid the turmoil.
In a dramatic turn, Richie pretends to be an FBI agent to lure Gina away from her people, leading to a confrontation where he threatens her life to compel Lempke into action. However, Billy intervenes, halting Richie's violent intentions. The warlock eventually agrees to meet them but clarifies that he can’t remove the curse; instead, he offers a chilling alternative: transferring the curse to another unsuspecting victim through a cursed pie.
Billy, torn between his conscience and desperation, refuses the dark suggestion of suicide that Lempke proposes. Instead, he returns home with the cursed pie, suspecting infidelity from Heidi meanwhile. In a tragic twist, he unwittingly enables her to consume a piece, leading to her demise.
Initially relieved to be free from his curse and what he perceives as marital betrayal, his relief turns to horror when he discovers that Linda has also consumed the pie. As he grapples with guilt over his daughter's fate, news breaks that Richie has been found dead under suspicious circumstances, hinting at a grim revenge from the Gypsies.
Overwhelmed by remorse, Billy prepares to accept his fate when Dr. Mike Houston unexpectedly arrives, inciting distrust yet again. In a cruel twist of fate, he invites the doctor in, contemplating whether the pie offers a chance to "die with dignity." As the tension crescendos, Billy stands at the precipice of both redemption and despair.
