Plot
Miniatures artist Annie Graham lives in northern Utah with her psychiatrist husband Steve and their two children, 16-year-old son Peter and 13-year-old daughter Charlie. The family attends the funeral of Annie's estranged mother, Ellen, at which Annie is surprised by the number of mourners in attendance. Charlie is especially affected by Ellen's death, becoming withdrawn, sullen and concerned about who will take care of her when Annie dies. While at school, Charlie beheads a dead bird with scissors and pockets the head. At a bereavement support group, Annie reveals that she and her mother had a strained relationship, but Ellen suddenly became a significant figure in raising Charlie after she was born. Meanwhile, Steve receives a phone call telling him that unknown perpetrators have vandalized Ellen's gravesite, which he keeps from Annie.
Peter is invited to a party, and Annie insists that Charlie accompany him. Left unattended by Peter, Charlie eats a piece of cake containing walnuts, triggering her severe nut allergy and sending her into anaphylactic shock. As Peter drives Charlie to the hospital, she leans out of the window to gasp for air. Peter swerves to avoid a dead deer in the road, causing Charlie to be decapitated by a telephone pole. In shock, Peter drives home and leaves Charlie's headless body in the back seat of Annie's car, which Annie discovers the following morning.
Following Charlie's funeral, Peter becomes reclusive and guilt-stricken, Annie becomes bitter and resentful toward Peter, and Steve tries to mediate peace between them. An unspecified amount of time later, Annie befriends fellow support group member Joan, who encourages her to communicate with Charlie's spirit via séance, demonstrating her own ability to contact her deceased grandson Louie when Annie reacts with skepticism. Joan explains that all members of the family must be present during the séance.
Later that night, Annie wakes Peter and Steve to conduct the séance. Objects begin to move, and Annie speaks in Charlie's voice until Steve throws water on her.
As supernatural forces begin haunting Peter, Annie suspects that Charlie's spirit has become vengeful and demonic. She sees threatening images manifest in Charlie's sketchbook, all of which involve Peter in some way. She then throws the sketchbook into the fireplace; her clothes catch fire at the same time, only to stop burning when she removes the book from the fire.
Annie goes through Ellen's belongings and finds a photo album showing Ellen as "Queen Leigh", the leader of a coven, and Joan as one of her acolytes. Another book describes the demon king Paimon, who, when summoned, brings wealth and knowledge, but prefers to inhabit a male host. In the attic, Annie finds Ellen's rotting, headless corpse and occultist runes drawn in blood.
While Peter is at school, Joan appears across the street and attempts to expel his spirit from his body for Paimon. In class, Peter is taken over by an unseen force and slams his head against his desk, breaking his nose. Annie tells Steve about the body she found in the attic and about Charlie's sketchbook and begs him to burn it, unable to bring herself to take her own life. When he refuses, believing Annie has gone insane, she snatches the book from him and flings it into the fire, only for Steve to burst into flames instead. Annie watches in horror, but is then taken over by the same force that possessed Peter.
As naked coven members gather both inside and around the house, Peter wakes after dark and finds his father's charred corpse, then notices one of the coven members in a nearby doorway. A possessed Annie pursues him through the house. He attempts to hide in the attic; Annie follows him and beheads herself with a length of piano wire as coven members appear. Horrified, Peter jumps out of the attic window and is seemingly killed by the fall. A light enters his body, reviving him. Now displaying Charlie's mannerisms, he follows Annie's floating corpse into Charlie's treehouse, where Joan and the other coven members, as well as the headless corpses of his mother and grandmother, are worshipping a mannequin with Charlie's crowned, severed head placed on it. Joan removes the crown and places it on Peter's head, addressing him as Charlie. She then proclaims him Paimon, telling him the coven have "corrected his female body" and given him his preferred male host. The coven hails Peter as King Paimon as he silently observes.
