Douglas "Dougie" Whooly, Alexander Brickel, is a nine-year-old boy whose mind lives in a video game where he plays Satan's little helper. His world tilts when his sister Jenna Whooly, Katheryn Winnick, comes home from college for Halloween, bringing along her boyfriend Alex Martin, Stephen Graham. The reunion quickly stirs trouble, and a rift opens between Dougie and Jenna that unsettles everyone in the house.
One day Dougie discovers a man in a cheap costume arranging a corpse on a lawn as if it were a spooky prop. The boy naively believes the figure is Satan and pleads for help to send Alex to Hell, a request the man eggingly agrees to, nudging the plot toward the grim Halloween night. Meanwhile, Alex proposes a macabre bonding idea: dress as Satan for Halloween to get Dougie on board with the game they’ve found themselves in. When Dougie returns home, he tries to lure Alex into the basement where Satan supposedly waits, but the plan backfires and Dougie changes tactics.
The plan shifts again: Dougie and Satan head out to shop for a true Satan costume, intending to ambush Alex. Instead, Alex survives and the two end up leaving with a haul of candy and tools, while Satan starts a terrifying spree that ends with the death of a market bagger after a chaotic confrontation with a shopping cart. As the night spirals, Satan stalks through the island, attacking several more people, and even reaches Alex's estranged father, Vernon Martin, Dan Ziskie before the duo slips away from the scene of the crime.
Back at home, Dougie brings Satan inside, and the household rapidly accepts the supposed bond between Alex and Satan as a sign of unwavering devotion. Satan's unnerving presence—his muteness, his forceful intimacy with Jenna—piles onto Dougie’s already fragile mental state, and Jenna begins to worry that Alex has gone too far in the costume’s world. The night devolves into a washing of costumes and danger: Dougie’s family discovers that Satan is not simply a role but a dangerous force masquerading as the boy’s brother-in-law.
A cascade of misdirection follows: Dougie’s father returns home, and Satan murders him; the family’s mother is kidnapped, pulling Jenna and Alex into a dire chase through a house that seems to morph with every new disguise Satan wears, including a Jesus costume used to lure Dougie back inside and to keep him from realizing the deadly truth. In a harrowing string of costume switches and false identities, the killer manipulates every moment to stay one step ahead.
The endgame arrives when Jenna and Mrs. Whooly confront the nightmare as truths blur: the killer has donned multiple guises, and what looked like Alex’s devotion was really a homicidal impostor. In a devastating twist, Jenna and Mrs. Whooly unwittingly kill Alex, believing him to be the culprit, while the actual Satan Man closes in on the house. The killer sprays a 6 on their home beneath the address 66, transforming it into 666, and enters the house in ominous silence. As the final moment unfolds, Dougie offers a wary explanation to Jenna: it’s so Satan won’t hear him.
The climax leaves a chilling resonance: a family is fractured by a child’s fixation and a nightmarish masquerade that blurred every line between play and reality. The sinister figure, masked by costume after costume, remains unseen until the last, a haunting reminder that the boundary between games and real danger can vanish in the blink of an eye.
