Annie Graham is a miniatures artist who lives in Utah with her husband, Steve, their 16-year-old son, Peter, and their eccentric and reserved 13-year-old daughter, Charlie, who is occasionally seen to make clicking noises with her tongue. At the funeral of her secretive mother, Ellen Leigh, Annie delivers a eulogy explaining their fraught relationship and her mother’s extremely private life. A week later, Steve is informed that Ellen’s grave has been desecrated, while Annie sees an apparition of Ellen in her workshop. At a support group for the bereaved, Annie reveals that the rest of her male family members suffered from mental illness that resulted in their deaths and how Ellen was a significant figure in raising Charlie.
To attend a party, Peter lies that he is going to a school event, and Annie forces him to take Charlie with him. Unsupervised, Charlie eats a cake containing nuts (to which she is allergic) and falls into anaphylactic shock. As Peter drives her to hospital, Charlie, struggling to breathe, leans out of the window for air. Peter swerves to avoid a dead deer and Charlie is decapitated by a telephone pole. In shock, Peter silently drives home and leaves his sister’s headless corpse in the car for their mother to discover the next morning. The family grieves following Charlie’s funeral, heightening tensions between Annie and Peter and causing Annie to behave coldly towards Steve. Peter is plagued by Charlie’s presence around the house.
Annie is befriended by a support group member, Joan. Annie tells her she used to sleepwalk and recounts an incident in which she awoke in Peter’s bedroom to find herself, Peter, and Charlie doused in flammable paint thinner with a lit match in her hand. Joan teaches Annie to perform a séance to communicate with Charlie. Annie convinces her family to attempt the séance. Objects begin to move and break, terrifying Peter, and Charlie seemingly possesses Annie until Steve splashes her with water. Annie suspects that Charlie’s spirit has become malevolent. She throws Charlie’s sketchbook into the fireplace but her sleeve also begins to burn. She retrieves it, after which the fire on her sleeve dissipates, and heads to Joan’s apartment for advice, but Joan is nowhere to be found.
Annie notices that Joan’s welcome mat resembles her mother’s craftwork. She goes through her mother’s possessions and finds a photo album linking Joan to Ellen, and a book with information about a demon named Paimon, who wishes to inhabit the body of a male host. Then, in the fly-riddled attic, Annie discovers Ellen’s decapitated body below strange symbols written in blood. At school, Peter sees his reflection in a glass cupboard door smiling at him, and inexplicably slams his head against his desk, breaking his nose and is brought home. Annie shows Steve her mother’s body and the sketchbook. Annie begs Steve to burn the sketchbook so she can sacrifice herself to stop the haunting, but Steve assumes she has gone mad, accusing her of desecrating Ellen’s grave herself. When Annie throws the book into the fireplace, Steve bursts into flames and burns to death. An ethereal light goes into a horrified Annie, possessing her.
Peter awakens in his bed and travels downstairs to find his father’s body. A possessed Annie then chases him into the attic, which is decorated with occult imagery. Levitating, Annie beheads herself with a piano wire as naked coven members look on. In fright, Peter jumps out the window and falls to the ground outside. Shortly after, the light that had entered Annie now enters Peter, waking him. He follows Annie’s levitating corpse into Charlie’s treehouse, where Charlie’s crowned, severed head rests atop a mannequin. Joan, other coven members, and the headless corpses of Peter’s mother and grandmother bow to him. Joan, addressing Peter as Charlie, swears an oath to him as Paimon, stating that he has been liberated from his female host, and is free to rule over them as their king. The coven members bow to Paimon in the treehouse, resembling one of Annie’s miniature replicas.