Ten-year-old Susan Walker Farrell attends her mother's funeral with her aunt Cora Nomed, who is appointed as her guardian. After the funeral, Susan slips into the family's mausoleum tomb and witnesses a demonic force that kills a vagrant in the cemetery. The encounter binds her to an ancestral demon tied to her lineage.
Twenty years later, Cora worries about Susan, now thirty and married to Oliver Farrell, on the anniversary of her mother's death. Susan and her husband go out for dinner, and Susan is attacked by a drunk patron outside. He enters his car, which Susan causes to burst into flames, killing him. Oliver is summoned to New York City for work, leaving Susan home alone. She seduces their landscaper, Ben, before killing him after sex with a hand rake. Oliver returns shortly after, and Susan tells him she gave Ben the night off from work.
The next day, Cora arrives at the house to bring Susan paperwork pertaining to the familial inheritance she is due at age 30. In the house, she finds Susan grossly deformed in the figure of a demon. Susan causes Cora to levitate over the staircase before breaking open her chest. That night, Oliver calls Dr. Simon Andrews, alarmed, claiming to have seen Susan in a disfigured state. Susan interrupts the call, appearing entirely normal. The following morning, the Farrells' housekeeper Elsie finds Susan's room glowing green and witnesses her in her deformed state, and flees the house in terror.
At the urging of Oliver, Susan visits Dr. Andrews, who tapes a hypnosis session with her in his office. Initially she reverts to a childlike state before becoming possessed by the demonic entity. Andrews ends the session prematurely, and Susan reverts to her normal self. Disturbed, Andrews consults his colleague, Dr. Roni Logan. The following morning, Susan kills another landscaper sent to her house. That night, Oliver returns home and finds blood splattered on the kitchen telephone.
Meanwhile, Andrews and Logan begin researching demonic possession, convinced that Susan's problem is supernatural and beyond the help of medicine. After analyzing a journal kept by Susan's grandfather, Andrews informs Oliver that the Nomed family is subject to a curse in which firstborn daughters fall prey to a demon. Susan visits a shopping mall alone, and steals a painting from an art gallery. When the gallery owner confronts her, she causes him to levitate over an atrium before he falls multiple stories, and his body is impaled on a sculpture below.
Andrews retrieves a crown of thorns from the Nomed family mausoleum, which, according to Susan's grandfather's journal, will expel the demon. Simultaneously Susan transforms into a fully formed demonic state at home, and brutally kills Oliver. Andrews arrives shortly after and manages to crown Susan, banishing the demon to the mausoleum. Andrews brings Susan to the mausoleum, and she uses the crown of thorns to banish the demon back to its tomb. Susan begins to cry as she now remembers killing her Aunt Cora and husband Oliver.
Before they depart, Andrews instructs a gravekeeper to keep the mausoleum sealed from the public. As they drive away, the gravekeeper laughs maniacally. It turns out the gravekeeper is actually Ben, the landscaper from earlier in the film.
