It's Christmas Eve in 1971, and the Chapman family—a father, mother, and their two young sons, Billy, who is just 5 years old, and infant Ricky—are driving down the empty roads of Utah in their station wagon, accompanied only by the sound of the radio. They arrive at a state mental hospital to visit their grandfather, who is gravely mentally ill. Although he poses no danger, he is unable to care for himself and doesn’t even seem to recognize their presence. Leaving Ricky with him, the parents step away to discuss matters with the doctors. In this brief moment alone, Grandpa suddenly turns to Billy with a smile and divulges a disturbing secret about Santa Claus: he only rewards good children with gifts and punishes the naughty ones. This revelation terrifies Billy, who is well aware of his bad behaviors throughout the year. When his parents return, it's already dark, and they head back home.
On the ride home, Billy, haunted by Grandpa's words, innocently inquires if Santa ever punished his mother as a child. Her reaction is swift and panicked, dismissing Grandpa as a "silly old fool," but this only increases Billy's fear because he has been taught it’s wrong to speak ill of elders—and he worries Santa Claus might punish his mother too.
Meanwhile, not far down the road, a man dressed as Santa Claus is robbing a gas station at gunpoint. In a violent struggle, he kills the clerk and drives off with a mere $31. As fate would have it, the Chapman family encounters this criminal thinking he is merely a man heading to a holiday party with car trouble. Billy, confused and terrified, believes this Santa has come to deliver punishment. In a chilling turn, the man pulls a gun, shoots the father dead, and subsequently assaults the mother. The tragedy culminates with Billy fleeing into the bushes, witnessing horrors that would forever haunt him.
Fast forward to Christmas 1974, it is revealed that both boys survived and are now living at a Catholic orphanage. Billy, at 8 years old, struggles with traumatic memories, especially during the festive season. His gruesome Christmas drawing results in punishment from the Mother Superior, who doesn’t grasp the depth of Billy’s trauma. Sister Margaret, a compassionate nun, senses the danger in Billy's mental state but is overridden by the harsh discipline of the Mother Superior.
In a haunting chain of events, Billy experiences flashbacks filled with horror when he accidentally witnesses two older children engaging in inappropriate behavior. This memory serves as a trigger, heightening his emotional instability. After a series of misguided punishments, Christmas arrives, bringing dread in the form of a man in a Santa suit, which terrifies Billy, leading to further emotional turmoil.
By spring 1984, we see Sister Margaret after a decade. Billy is now 18 and begins working at a toy store managed by Mr. Sims. He appears to be doing well until Christmas approaches, stirring memories of his past. An unfortunate turn of events forces Billy to don a Santa suit when the designated Santa injures himself—a decision that proves to be disastrous.
On Christmas Eve, a company celebration takes a dark turn as Billy, already on edge, confronts the abuse unfolding in the back room of the store. Overwhelmed by his horrific past, he snaps, believing himself to be Santa capable of delivering punishment. This sets off a series of violent encounters culminating in a gruesome murder spree at the toy store.
The chaos expands into the neighborhood as Billy, still donning the Santa suit, approaches innocent households, leaving carnage in his wake. Sister Margaret, aware of the danger Billy poses but unable to prevent him, faces the consequences of her earlier decisions.
As the police search for Billy, he manages to infiltrate the orphanage, confronting the now wheelchair-bound Mother Superior. In a heart-stopping climax, as he raises his weapon to punish her for the years of torment and punishment she inflicted upon him, he is met with gunfire from a police officer. In his dying breath, Billy assures the frightened children, “You’re safe now. Santa Claus is gone,” leaving a haunting hint at the legacy of violence that may continue with his younger brother, Ricky, who ominously remarks, "Naughty!"
