After the death of her secretive mother Ellen, Annie Graham tries to keep her family together. Her husband Steve wants everything to feel normal, her teenage son Peter wants to be left alone, and her strange 13-year-old daughter Charlie - who makes odd clicking sounds with her tongue - was always Ellen's favorite. At the funeral, Annie notices people she does not recognize, and we soon learn Ellen had a hidden life.

Annie, who builds miniature models for a living, attends a grief support group and reveals her family's dark history: her father starved himself to death, and her brother hanged himself as a teenager, accusing their mother of "putting people inside him." She also admits to a terrifying sleepwalking episode where she once woke up standing over her sleeping children with paint thinner and a lit match.

Peter is forced to take Charlie to a party, where she eats a cake containing nuts - a food she is dangerously allergic to. Racing her to the hospital at night, Peter swerves to avoid something in the road, and Charlie leans out of the window gasping for air. A telephone pole, marked with a strange symbol, takes her head. In shock, Peter drives home and leaves her body in the back seat. The next morning, Annie finds her daughter's headless body in the car - the moment the family truly breaks.

Months later, Annie meets Joan, a friendly woman from the support group, who teaches her how to hold a séance to contact Charlie. The ritual goes horribly wrong. Glass moves on its own, Annie speaks in Charlie's clicking voice, and Steve begs her to stop. Annie then discovers the truth: Joan knew Ellen. They were part of the same cult, and the symbol on the pole is the sigil of Paimon, a demonic king from old occult texts.

The cult's plan was always to give Paimon a male body - Peter. When Annie kept Peter away from Ellen, they used Charlie as a temporary vessel instead. Her death was no accident; it was the first step in moving Paimon into his true host. Possessed, Annie chases Peter through the house. Steve bursts into flames. Cornered in the attic, Peter watches in horror as his mother, levitating, cuts off her own head with a piano wire. Her headless body floats up into the treehouse.

Peter throws himself out of the attic window. A glowing light enters him. Drawn by the clicking sound, he walks to the treehouse - where naked cult members bow to him. Joan places a crown on his head. Charlie's spirit watches. The ritual is complete: Paimon has a new body.

The ending reveals that everything was planned from the start. Ellen led a cult that worships Paimon, a king of demons described in real 17th-century occult books. Paimon needs a male host, and Ellen always wanted her grandson Peter. When Annie kept them apart, the cult used Charlie as a stopgap - which is why her death was engineered through her nut allergy and the marked pole.

Each death moves the plan forward. Charlie's beheading removes her from the body so Paimon can travel. Annie's self-decapitation is the final sacrifice, offered by the demon itself using her body. When Peter jumps from the attic window, his spirit leaves his body - giving Paimon a way in. The light that enters him and the crown placed on his head complete the transfer.

The final click confirms it: Peter is gone, and Paimon is home. Even the film's first shot - pulling out of one of Annie's miniature houses - was a hint. We were watching tiny people inside a model all along, moved by hands they could not see.