The story begins with the viewer looking out from a window in a workshop to a tree house, then turning and zooming in to a bedroom in a dollhouse that is in the workshop. Steve Graham wakes his teenage son Peter and 13-year-old daughter Charlie for their 78-year-old grandmother Ellen Taper Leigh's funeral. Steve finds Charlie sleeping in the tree house.
Steve's wife Annie, an artist who sculpts miniature dioramas, delivers the eulogy at her mother's service. Charlie makes a clucking noise while drawing a strange sketch during the speech.
Annie talks to Charlie about Ellen at bedtime that night. Charlie claims that her grandmother always wished Charlie were a boy. To Annie's confusion, Charlie also wonders aloud who will care for her now that Ellen is dead. Annie later sees a haunting vision of Ellen after looking through a memory book while in Annie's workshop.
A bird dies by flying into one of Charlie's classroom windows at school. Charlie goes outside and cuts off the bird's head. A woman across the street waves at Charlie.
Annie begins researching apparitions. Steve receives word from the cemetery that Ellen's grave was desecrated, but he decides to not tell his wife.
Annie tells Steve she is going to a movie, but actually attends a grief counseling support group. When she arrives at the meeting it is dark outside. Annie openly discloses her mother's mental health issues including the dissociative identity disorder and dementia.
Charlie sees a strange light in her bedroom. Gallery owner Silvia Archer contacts Annie about progress on her new works, which include a piece featuring Ellen.
Peter asks his mother if he can go to a party where he hopes to see Bridget, a classmate he is interested in. Annie asks Peter if he invited his sister to go with him, since he claimed it was a party related to their school.
Charlie experiences a vision of her grandmother surrounded by fire. Charlie makes her clucking noise when she is shaken out of her trance. Charlie tells Annie that she wants Ellen. Annie forces Peter to take Charlie with him to the party.
Flustered at having to monitor his sister, Peter blows off Charlie so he can smoke marijuana with Bridget and their friends. Many of women at the party are wearing long flannel-style shirts. Left unsupervised, Charlie unknowingly eats chocolate cake containing a substance to which she is allergic. Charlie begins choking as she experiences an anaphylactic reaction.
Peter carries his sister to his car and rushes her toward the hospital along a dark country road. Charlie sticks her head out the window in an effort to breathe better. Peter swerves to avoid an animal in the road. Charlie is decapitated when her head violently hits a utility pole.
After sitting and staring in entranced shock, Peter drives home in a calm daze. Annie comes outside when it is light outside and is horrified to find her daughter's headless body in the car's backseat.
The family holds a funeral for Charlie. Steve looks through Charlie's sketchbook of drawings. Peter experiences a panic attack while smoking marijuana, before biking home from school. Peter arrives at home in the dark. Annie grieves alone while sitting in the car in the driveway.
Annie drives to her grief support group meeting, but decides to turn around while still in the parking lot. It is dark when she arrives. However, before Annie can leave, fellow group member Joan spots Annie and stops her to talk. After hearing about Charlie's death, Joan confides in Annie about the loss of her own child and grandson.
When Annie returns home, Steve makes a pass at her, but Annie rebuffs him. Annie sleeps in the attic. Peter hears Charlie's clucking noise and sees what he thinks is a vision of his dead sister in his room, but it appears to be his own hoodie in the corner.
Annie visits Joan at Joan's apartment. Annie tells Joan about a sleepwalking incident in which she doused Peter and Charlie and herself from head to toe in paint thinner before waking up to find herself preparing to light a match. From her body language, Annie implies that the matches were in her left hand and can of paint thinner in her right. Annie explains that her relationships with her children were never the same afterward.
Steve finds Annie constructing a disturbing diorama of the scene where Charlie died. Steve, Annie, and Peter have an awkward dinner during which Annie blames her son for Charlie's death. Peter responds by reminding Annie that she was the one who forced Charlie to go to the party.
Annie runs into Joan at an art supply store. Joan excitedly explains to Annie that she attended an open séance that changed her skepticism about psychics. Joan tells Annie that a medium was able to conjure her dead grandson Louis and taught Joan how to conduct a séance as well. Joan has a chalkboard in the trunk of her car.
Joan invites Annie over to witness a séance firsthand. Joan seemingly makes contact with Louis, who uses a glass and a chalkboard to communicate. Joan assures Annie that she can conduct a similar conjuring herself by using a personal item from the deceased, reciting a cryptic incantation, and making sure that her entire family is in the house during the summoning. Annie hears a clucking sound while driving home afterward.
Annie wakes that night to find a swarm of ants leading to Peter's dead body. Annie wakes from a sleepwalking trance over her son's bed, prompting a conversation with Peter. Peter asks why Annie is seemingly scared of him. Annie involuntarily confesses that she never wanted to be Peter's mother and tried to have a miscarriage. Annie suddenly wakes to discover she was experiencing a vision within a vision.
Annie recites Joan's incantation with Charlie's sketchbook while Steve and Peter sleep. Claiming she summoned Charlie, Annie excitedly wakes her husband and son for another séance. Charlie seemingly possesses Annie. Steve snaps Annie out of her trance by dousing her with water as Peter cries from confused fright.
During school, Peter sees the same strange light that Charlie previously saw in her bedroom. Peter notices that his reflection looks back at him with a different expression.
Steve admonishes Annie for Peter becoming convinced that a vengeful spirit is threatening him. Annie trashes her studio in frustration when she accidentally breaks a tiny model chair, after another voicemail from her gallery pressures her about providing new pieces.
Charlie's spirit supernaturally draws in her old sketchbook. Peter sees a vision of his dead sister in the corner, and her head falls off turning into a recreation ball on the floor, before being choked in his bed. Peter accuses his mother of sleepwalking and attacking him again. Annie advises Peter not to tell Steve what happened. Annie goes on to explain that something supernatural is happening in the house and she is the only one who can stop it. The window above Peter's bed has a mark that looks similar to the one in Charlie's classroom when struck by the bird.
Realizing that the spirit she summoned is malevolent, Annie throws Charlie's sketchbook into the fireplace. Annie's arm mirrors the burning book by also catching fire, forcing Annie to rescue the book.
Annie returns to Joan for help, but no one's there and she does not go inside Joan's residence. The camera shows us Joan's place is decked out in witchcraft paraphernalia, including a photo of Peter inside a ceremonial triangle and a symbol Annie recognizes from family photos.
Annie learns that the symbol is associated with the demon Paimon, one of the kings of Hell. Annie also finds photos of Joan with Ellen, revealing that Joan and Annie's mother were in the same coven devoted to gaining riches by conjuring Paimon into a male body. Annie discovers Ellen's headless corpse in her house's attic.
Peter hears Joan shouting, "I expel you" at him from a distance at school behind a fence. During class, Peter hears Charlie's cluck. Peter acts possessed and suddenly bashes his head into his desk, snapping out of his trance with cries of terror and pain.
Annie stands in the pouring rain below the tree house with Ellen's scrapbook, and we see that Peter's room behind her in the real house does not exist.
Steve brings Peter home. Annie approaches the car, and is dry with no sign of the downpour. Annie tells Steve that Ellen's corpse is in their attic, but it is now decapitated. Annie also shows Steve the photographs where Joan and Ellen are wearing the seal of Paimon. Annie explains that their family became cursed when she tried contacting Charlie. Annie also explains the connection to Charlie's sketchbook, adding that Steve needs to destroy it in order to save Peter. Peter sleeps in his room in the background and is not awakened by the conversation.
Disbelieving her wild claims, Steve accuses Annie of digging up Ellen's grave. When Steve refuses to burn the sketchbook, Annie throws it back into the fire, even though she presumes doing so will kill her. Instead, Steve spontaneously combusts.
With his possessed mother hovering in the corner above his bed, Peter gets up to search the house. When Peter leaves his room the ladder to the attic is withdrawn missing. Peter finds his father's charred corpse. Possessed, Annie chases Peter to the attic. The ladder to the attic is now down. Annie jumps up and furiously pounds her head on the attic door after Peter climbs the ladder and retracts it into the upstairs ceiling.
In the attic, Peter finds flies, candles, and a photo of his face with the eyes punched out. Ellen's body is gone. Annie suddenly hovers above Peter before severing her own head. Confronted by this horror and three undressed devil worshipers, Peter jumps out the window.
Peter's head hits the ground below, which seemingly knocks him out. Peter rises after the oddly glowing light seen previously hovers around his body. Peter follows his mother's headless corpse as it floats into the tree house.
An assembly of mostly devil worshipers in various states of undress greets Peter inside the tree house. There is one woman with long hair in a bathrobe. Charlie's decapitated head sits atop a statue of Paimon. Peter looks around with a dazed, flat expression and we're shown Annie and Ellen's headless bodies lie bowing on the floor, in front of the statue. Joan's voice calls Peter 'Charlie' as a woman crowns him, but welcomes Peter as Paimon while the coven hails the demon's arrival. The story ends with a shot of a model tree house filled with dolls that look like Peter, the coven, and the headless Annie and Ellen.