Linda Blair stars as Rachel, teenage daughter of Leslie and Tom Bryant, a typical, middle-class Californian couple. When Leslie's sister, brother-in-law, and their housekeeper are tragically killed in a car crash, the Bryant's agree to look after their now orphaned niece, Julia, for the summer. Although the family haven't seen Julia in ten years, they immediately warm to her, with Rachel even agreeing to share her bedroom with her cousin. But it's not long before the initially shy Julia begins to exert a powerful influence over the family. Perturbed by a series of strange events involving Julia, Rachel turns detective, eventually uncovering the terrifying truth - but will anybody believe her?