Julia Trent is a teenage girl who becomes an orphan after a car accident on the East Coast that claims her parents and their housekeeper. She is taken in by her Aunt Leslie Bryant and Uncle Tom Bryant at their California ranch, joining their children, Rachel Bryant and Peter Bryant, along with their adolescent son, Bobby, as the new presence in the household.
Initially, Rachel Bryant is thrilled to have a girl her age in the house and even offers to share a room, but Julia comes across as painfully shy, and the family notices a distinct East Coast accent that doesn’t quite fit the laid‑back West Coast life. Seeking to belong, Julia undergoes a makeover and presents a more polished, sophisticated façade that hides a growing sense of unease beneath the surface.
The peace is fragile from the start: one day, Rachel’s horse Sundance suddenly lashes out and tries to trample Julia. The attack ends with Julia surviving, and she gradually works her way into the family’s daily rhythm, yet oddities continue to accumulate. A human tooth is found among Julia’s belongings, a photo of Rachel goes missing, and burn marks from Sundance’s hair appear in Julia’s dresser drawers. Soon after, Rachel is struck with a mysterious hive outbreak that prevents her from attending a school dance, so Julia steps into her place, borrowing a dress Rachel had sewn and accompanying Rachel’s boyfriend, Mike Gallagher. The planned night out becomes a turning point, as Julia’s presence deepens her ties to the household and to Carolyn Baker—Rachel’s friend who grows closer to Julia despite the strangeness surrounding her.
The day after the dance, Sundance is entered into a competition in which the horse becomes frightened and suffers a broken leg, leading a veterinarian to euthanize it. Troubled, Rachel seeks help from their neighbor, Professor Jarvis, who hints that the family may be dealing with black magic and warns that true witches cannot appear in photographs. This unsettling idea prompts Rachel to push for evidence, and she persuades her mother to take pictures of Julia in an effort to confirm or disprove the theory.
As Leslie plans a road trip, Rachel finds a map with burn marks and grows convinced that Julia intends to harm her mother. Her fears escalate when she also witnesses Julia making overt advances toward her father, deepening the sense that something terrifying is at work beneath the surface. Before Leslie can depart, Rachel develops the film and reveals a chilling truth: Julia’s image does not appear in any of the frames. Then, in a dramatic confrontation, Julia reveals herself to be Sarah Brown, the housekeeper in disguise, and not the cousin they believed her to be. A brutal struggle ensues, and Rachel manages to escape, locking the darkroom door behind her.
The chase spills into the night as Sarah, now described with an eerie, almost otherworldly intensity, pursues them, drawing dangerously close to Mike’s car. The pursuit ends when Rachel and Mike manage to reach her mother, whose car causes Sarah to drive off a cliff into a fiery explosion. In the aftermath, the Bryant family begins to work toward a semblance of normal life, even adopting a new horse to replace Sundance. Yet the film closes on an unsettling note, as a separate family welcomes Julia into their home under the guise of a nanny, leaving with the implication that the danger may not be entirely behind them.
