Eleven-year-old David Stanley sits on the stair landing of Westerly House, an old and largely unremodeled country home, while his stepmother, Molly, drives to pick up her twelve-year-old daughter, Amanda. David is the oldest of four Stanley children; his siblings are six-year-old Janie, a relentlessly talkative girl, and four-year-old twins Blair and Esther, whom everyone calls "Tesser." Their mother is dead, and David has a strong premonition that Amanda's arrival will bring drastic changes.
When Molly's car arrives, she rushes inside looking as though she might be crying, leaving Amanda sitting alone. Amanda emerges wearing dozens of tight braids, a bright shawl, a long black dress, and a shiny triangle on her forehead. In her new room, she reveals that the largest of her cages contains a crow she calls her "Familiar," a spirit in animal form that connects her to the supernatural. She has been studying the occult under the guidance of her older friend Leah. The crow, which she names Rolor, pecks her viciously; she has had it only a few days. Her father in Southern California bought it and secretly rearranged her travel plans to include a night with Leah, which partly explains the tension between Amanda and Molly.
Amanda's first dinner with the family is strained; she refuses to speak unless asked a direct question and gives long, cool stares before answering. David's father, Jeff Stanley, a geology professor, privately asks David to be patient, explaining that Amanda is upset about her parents' divorce and her mother's remarriage. Amanda's behavior is unpredictable: She treats David coldly near Molly but seeks him out when they are alone. In the garage loft, she confides that she hates Molly for divorcing her father, remarrying a man she considers poor, and moving to the country. When David questions whether her father truly wants her, Amanda shoves him over a rock.
Despite her hostility, Amanda invites the children to become her "neophytes," or learners in the occult. She tests them for psychic ability with playing cards, asking each child to guess whether a card is red or black. When Blair's turn comes, he says, "It's valentines. It's lots of little valentines." The card is the nine of hearts. Before anyone can react, the crow escapes its cage. Everyone dives to the floor except Blair, who calmly wraps his arms around the enormous bird and carries it back. When Amanda tries to approach the crow using a magic chant, it attacks her. She throws it into the cage, kicks the cage, and dismisses everyone.
Amanda designs nine "ordeals" that must be completed before an initiation ceremony. Each lasts a full day, and the rules cannot be explained to adults. The first requires that no metal touch their skin, turning meals into elaborate improvisations: Janie wears rabbit fur mittens, Esther uses a tiny plastic shovel, Blair eats with his fingers, and David dumps his dinner into a plastic bag in his lap. Subsequent ordeals include carrying a live reptile all day, wearing garlic and onion on a necklace, avoiding wood floors, and observing silence. Dad grows increasingly bewildered by the family's strange behavior.
The initiation takes place while Molly is away. Amanda transforms her room with blankets, a candle, and incense, and each child receives a spiritual name. When the children are blindfolded and asked to sacrifice treasured possessions, David discovers that Janie has given up their dead mother's pearl and opal ring. He demands it back, but Amanda insists it belongs to the spirits. Blair slips into Amanda's room and returns with the ring, saying he found it "behind the curtain." Amanda accuses Blair of lying, and David angrily leads the children out.
Amanda is cheerful the next day, planning a seance as if the fight never happened. Dad leaves for a three-week geology field trip. When the house's old electrical system fails, a retired handyman named Mr. Golanski arrives and mentions that the cupid's head on the banister was "damaged by the poltergeist," a German word meaning "noisy ghost." Amanda presses him for details and learns a famous poltergeist haunted the house long ago. At the library, she finds newspaper articles from 1896 describing showers of rocks, self-moving furniture, and the cupid's head being chopped off. The two Westerly daughters were suspected but never proven responsible, and the disturbances stopped only when they were sent to boarding school. Amanda insists David not tell Molly.
Amanda conducts a midnight seance. Rapping sounds come from the table, and a dim, human-shaped figure with glowing eyes appears in the closet. The younger children panic, and David rushes them out. Blair later says he heard the ghost talking rather than rapping and that it said something "about Amanda," but he refuses to elaborate.
Rocks begin appearing inside the house, escalating from scattered pebbles to flying stones that smash the milk pitcher at dinner. One night, Molly's large potted plant is sent tumbling down the stairs. Another night, a painting is smashed off the wall by a crystal paperweight. Molly calls her friend Ingrid for help. On the evening Molly drives to pick up Ingrid, her car breaks down, leaving Amanda and David alone with the sleeping children. Amanda is unusually warm and playful. After the children are in bed, an avalanche of objects cascades down the stairs, followed by Amanda's scream from the kitchen. David finds the stairs covered in rocks, and on the landing sits a wooden box containing the carved wooden head of the cupid, missing for over seventy years.
Amanda appears looking genuinely terrified and confesses to faking all the previous poltergeist activity: She rolled the planter, threw the paperweight, gathered rocks from the creek, and produced the seance rapping with a metal ring fitted around her big toe. The glowing figure was a cardboard cutout controlled by a string. She did it partly to force a move back to the city and partly "to get even" with everyone. But the box with the cupid's head was not her doing. She reveals she has already told Molly everything during a long private conversation.
The household stabilizes. Amanda spends more time with the children and with Molly, giving away her animals as gifts. Then David notices bruises on Blair's legs. Blair explains that a loose panel in his window seat revealed a hidden space containing the box. He tried to carry it downstairs and fell, sending everything crashing. David decides to keep Blair's discovery secret, reasoning that Amanda's belief in a real poltergeist has contributed to her transformation.
When Dad returns, he and Amanda have a long private talk, after which she begins speaking to him even in public. One morning, David sits on the landing gluing the cupid's head back onto the banister. Blair watches and says, "She'll like that," referring to a girl who used to live in the house and told him where to find the head because she wanted the cupid to have it back. When David asks if this was a ghost, Blair nods and says, "I think." He then chases his basketball down the stairs, refusing to say more. David is left wondering whether something genuinely supernatural guided Blair, a question that lingers every time he passes the cupid on the landing, its head reattached and sitting slightly crooked.