After their third child is stillborn, Kate and John Coleman's relationship is strained, especially as Kate is a recovering alcoholic. The couple adopt a girl named Esther from the local orphanage. Esther befriends the couple's deaf daughter, Max, but Daniel, Max's elder brother, refuses to accept her.
Esther injures her classmate Brenda, who had previously bullied her, evoking suspicions from Kate. The family is visited by Sister Abigail, the head of the orphanage, who warns that bad things happen in Esther's presence. As Sister Abigail leaves, she is confronted by Esther and Max. Esther kills Sister Abigail and warns Max not to tell anyone. Kate, growing increasingly suspicious of Esther, researches the girl's history and discovers that Esther used to reside in a mental hospital.
Max reveals to Daniel that Esther killed Sister Abigail. Esther, to keep Daniel from proving her guilt, attempts to kill him, but is stopped by Max. Daniel is hospitalized and Esther smothers him with a pillow, though the doctors manage to revive him. Kate attacks Esther, but the doctors sedate her. John takes Esther and Max home, where Esther attempts to seduce him. John, realizing Kate was right, tells Esther that he will return her to the orphanage. Meanwhile, Kate receives a call from the mental hospital, who reveals that Esther is an adult woman named Leena Klammer, who has a hormone disorder that stunted her physical growth. Leena pretends to be a young girl in order to be adopted into a family and seduce the father.
Leena murders John as Kate returns home. Leena wounds Kate and then sets off to kill Max. However, Kate knocks Leena unconscious and takes her gun. Kate and Max leave the house as the police arrive, pursued by Leena. Kate and Leena battle on a frozen pond, culminating in Max shooting the ice, shattering it and causing the two to fall into the water. Kate emerges from the water, along with Leena, who begs Kate not to let her die while hiding a knife behind her back. Kate angrily responds that she isn't Leena's mother and breaks her neck, killing her. Kate and Max are then met with the police afterwards.
- Vera Farmiga as Kate Coleman.
- Peter Sarsgaard as John Coleman.
- Isabelle Fuhrman as Esther Coleman.
- CCH Pounder as Sister Abigail.
- Jimmy Bennett as Daniel Coleman.
- Margo Martindale as Dr. Browning.
- Karel Roden as Dr. Varava.
- Aryana Engineer as Max Coleman.
- Rosemary Dunsmore as Grandma Barbara.
- Jamie Young as Brenda.