Social worker Emily Jenkins (Renée Zellweger) is assigned to investigate the family of ten year-old Lilith Sullivan (Jodelle Ferland), as her grades have declined and an emotional rift with her parents has emerged. Emily suspects that the parents have been mistreating Lilith, and proposes to her department to take the child away from her parents' custody. Emily's fears are confirmed when Lilith's parents try to kill her by roasting her in the oven at their home. Emily saves Lilith with the help of Detective Mike Barron (Ian McShane). Lilith 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 Lilith until a suitable foster family comes along. In the meantime, Lilith's parents, Edward and Margaret (Callum Keith Rennie and Kerry O'Malley), are placed in a mental institution.
Not too long after Lilith moves in, strange things begin to happen around Emily. Two weeks later, another of Emily'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, Lilith undergoes a psychiatric evaluation by Emily's best friend, Douglas J. Ames (Bradley Cooper). During the session, however, Lilith 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 by snapping his own neck.
Emily gradually becomes fearful of having Lilith in her home, so she heads to the mental asylum for answers from Lilith's parents. They tell her that Lilith is a demon who feeds on feelings, and that they tried to kill her in an attempt to save themselves. Lilith's father tells Emily that the only way to kill Lilith is to get her to sleep. Shortly after Emily leaves the asylum, both parents die in unusual circumstances. Lilith's mother is fatally burnt and her father is stabbed in the eye with a fork after attacking a fellow inmate through whom the voice of Lilith spoke. 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 Lilith. He arms himself at the police precinct to aid Emily in handling Lilith. However, he inadvertently shoots himself in the head with a shotgun when Lilith makes him imagine he is being attacked by dogs. After realizing all her most closest colleagues have been eliminated, this prompts Emily to serve Lilith tea spiked with sedative. While Lilith is asleep, Emily sets fire to her house, hoping to get rid of her. However, the girl escapes unharmed. A police officer escorts Emily and Lilith 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 Lilith. Instead, Lilith forces Emily to relive her childhood memory of her mother driving fast in a rainstorm. Emily fights through the memory, telling herself that it is not real. The image fades, and Emily asks Lilith if she is afraid. Lilith now appears afraid as she knows that Emily is no longer afraid. Emily then drives the car off a pier. As the car sinks, Emily struggles to lock Lilith (now in demon form) in the trunk by folding the rear seats against her. Emily then exits the car, but as she swims away, Lilith grabs her leg after punching a hole through the car's left tail light section. Emily struggles to break free until Lilith finally lets go as the car continues to sink. She climbs back ashore, relieved to be rid of her.