Social worker Emily Jenkins (Renée Zellweger) is assigned to investigate the family of 10-year-old Lillith Sullivan (Jodelle Ferland), as her grades have declined and an emotional rift with her parents has emerged. Emily suspects that the parents have been abusing Lillith, and proposes to her department to take the child away from her parents' custody. Emily's fears are confirmed when Lillith's parents try to kill her by roasting her in the oven at their home. Emily saves Lillith with the help of Detective Mike Barron (Ian McShane). Lillith is originally sent to a children's home but she begs Emily to look after her instead. With the agreement of the board, Emily is assigned to take care of Lillith until a suitable foster family comes along. In the meantime, Lillith's parents (Callum Keith Rennie and Kerry O'Malley) are placed in a mental institution.
Not too long after Lillith moves in, strange things begin to happen around Emily. Two weeks later another of Jenkins's cases, a boy named Diego (Alexander Conti), suddenly murders his parents, and Barron informs Emily that somebody phoned Diego from her house the night before the crime. As she is suspected of involvement in the incident, Lillith undergoes a psychiatric evaluation by Emily's best friend, Douglas J. Ames (Bradley Cooper). During the session, however, Lillith turns the evaluation around, asking Douglas what his fears are and subtly threatening him. That night while studying he receives a strange phone call in his apartment, Douglas is panicked by the sight of a mass of hornets coming out of his body and kills himself in his bathroom.
Emily gradually becomes fearful of having Lillith in her home, so she heads to the mental asylum for answers from Lillith's parents. They tell her that Lillith is a demon who feeds on feelings, and that they tried to kill her in an attempt to save themselves. Lillith's father tells Emily that the only way to kill Lillith is to get her to sleep. Shortly after Emily leaves the asylum, both parents die in unusual circumstances. Lillith's mother is fatally burnt and her father is stabbed with a fork.
Barron initially thinks Emily should seek psychiatric help, but is later convinced when he receives a strange phone call in his home from Emily's cellphone, which is being used by Lillith. He arms himself at the police precinct to aid Emily in handling Lillith. However, he inadvertently shoots himself when Lillith makes him imagine he is being attacked by dogs. That night, Emily has Lillith drink tea spiked with sedative. While Lillith is asleep, Emily sets fire to her house, hoping to get rid of her. However, the girl escapes unharmed from the burning house.
The police offer to escort Emily and Lillith to a temporary place to sleep. As Emily is following the police cars, she suddenly takes a different route and drives her car at a high speed, hoping to bring fear to Lillith. She then drives the car off a pier. As the car sinks, Emily struggles to lock Lillith (now in the form of her demon self) in the trunk by folding the rear seats against her. Emily then exits the car, but as she swims away, Lillith grabs her leg after punching a hole through the car's left tail light section. Emily struggles to break free until Lillith finally lets go as the car continues to sink. She climbs back ashore, relieved to be rid of Lillith.