John and Kate Coleman are a couple whose marriage is on the rocks after their third child is stillborn; Kate, a recovering alcoholic, takes the loss particularly hard. She and John decide to adopt Esther, a nine-year-old Russian girl, from a local orphanage. While their 5-year-old deaf daughter Max warms to Esther, their 12-year-old son Danny is less welcoming.
One night, Esther walks in on John and Kate having sex, and Kate becomes alarmed when Esther expresses far more knowledge of sex than expected of a child of her age. Max and Danny then witness Esther exhibit hostile behavior, from killing an injured bird to injuring a classmate out of revenge for bullying her. Sister Abigail, the head of the orphanage, pays the Colemans a visit and tells them that tragic events happen when Esther is around, such as a fire that killed her last adoptive family. When Sister Abigail leaves, Esther causes her car to swerve off the road and bludgeons her to death with a hammer. She then forces Max to hide the body, and hides the evidence in Danny's treehouse, threatening to castrate him if he blabs to Kate and John.
As Kate becomes further convinced of Esther's unusual behavior, John believes she's being paranoid and suggests Esther does something nice for her. Esther then gives Kate a bouquet of flowers taken from her stillborn daughter's grave, much to her horror. Kate grabs Esther's arm, and in distress, asserting she did it on purpose; Esther then snaps her arm, and lies that Kate abused her, causing more strain on the Coleman's marriage. The next day, Esther releases the brake in Kate's car with Max inside and points out the bottles of wine she found in the kitchen. Kate's therapist suggests she goes back to rehab, and John threatens to take custody of their children if she refuses. Kate then phones the Saarne Institute in Estonia, which she then finds out is a mental hospital, and the orphanage Esther claims to be from has no record of her.
When Danny learns of Sister Abigail's death from Max and searches the treehouse, Esther locks him and sets the treehouse ablaze to try and kill him, but Max stops her; in his attempts to escape, Danny is knocked out, and hospitalized. When Esther tries to smother him to death, he is revived by doctors. In a rage, Kate hits Esther in the face before the orderlies sedate her. That night, Esther dresses provocatively and tries to flirt with John, who realizes Kate was right and tries to send her back to the orphanage.
Dr. Varava, the head of the Saarne Institute, phones Kate and tells her that Esther's real name is Leena, and she is actually an adult woman who suffers from hypopituitarism, a condition which means she maintains a child-like appearance, and has spent most of her life pretending to be a young girl; Leena was also one of the most violent patients at the Institute, she has killed at least seven people, and the bows she wear hide the scars she suffered from breaking out of her straitjackets. After her attempts to seduce John fail, Leena removes her disguise, trashes her room and fatally stabs him.
Kate arrives home and Leena tries to shoot her, wounding her arm. After Leena tries to shoot Max, Kate breaks through the greenhouse roof and knocks Leena out. Kate and Max flee as the police arrive, but Leena attacks Kate near a frozen pond. Max tries shooting her, but breaks the ice instead, sending Leena and her mother underwater, as Kate resurfaces, Leena tries to drown her, reverting to her Esther persona. Kate retorts she isn't her mother and kicks her in the face, breaking her neck and sending her underwater. Kate and Max embrace as the police meet them.
