In January 2007, Leena Klammer, a 31-year-old psychiatric patient suffering from hypopituitarism, which makes her resemble a 9-year-old child, meticulously plans her escape from the Saarne Institute. By seducing and murdering a guard, she manages to flee while hiding inside the vehicle of an art therapist named Anna. After breaking into Anna's home and subsequently killing her, Leena seeks out information on missing American girls, discovering her uncanny resemblance to Esther Albright, a wealthy girl who vanished in 2003. Posing as a lost child, she meets a Russian officer and introduces herself as Esther, claiming her parents are in the United States.
Meanwhile, in Darien, Connecticut, Allen Albright and his wife Tricia, who have struggled to come to terms with their daughter's disappearance, are informed by Detective Donnan about “Esther” being found. Tricia flies to the U.S. embassy in Moscow for an emotional reunion with Esther. However, doubts begin to cloud Tricia's thoughts as she notices discrepancies, such as Esther's lack of memory regarding her grandmother’s death and an astonishing improvement in her artistic skills.
As Leena and Allen bond over their shared passion for painting, Leena becomes infatuated with him and plots to drive a wedge between him and Tricia. During a charity gala hosted by Tricia, Donnan uncovers a vinyl record belonging to Esther that bears Leena's fingerprints. In a tense confrontation, Leena brutally attacks Donnan, stabbing him repeatedly as Tricia arrives just in time. Instead of intervening, Tricia shoots Donnan dead, revealing her awareness of Leena’s deception — Tricia knows Leena is not Esther, who actually died in an incident with her older brother, Gunnar.
In a shocking twist, Tricia discloses that she only welcomed Leena to mend Allen’s broken heart following their daughter's disappearance. As Leena reveals her true identity, the two conspire to dispose of Donnan’s body in a hidden spot where the real Esther is buried, skillfully fabricating a story to pass off his disappearance as a vacation.
Understanding the precarious nature of their situation, Tricia and Gunnar decide they must continue to support Leena's ruse for Allen’s sake, but a brewing tension between Leena and Tricia simmers beneath the surface. One night, during dinner, Tricia attempts to poison Leena, but her plan backfires when Leena refuses to eat and instead disposes of the poisoned food, which a rat she feeds it to subsequently dies from. Enraged, Leena retaliates with a green smoothie laced with the rodent's carcass.
When Allen leaves for a meeting regarding an art gallery, Leena seizes the opportunity to eliminate both Tricia and Gunnar by attempting to push them in front of an oncoming train. Her plan unwittingly gets thwarted by a commuter. In a desperate attempt to escape, Leena tries to steal Tricia’s car but is soon apprehended by the police.
Later that night, captured once more at the Albright residence, Tricia and Gunnar determine that Leena poses too great a risk to their family and decide she must die. In a violent struggle, Tricia tries to stage a suicide for Leena, but the tables turn as Leena fights back. After a brutal clash, Gunnar finds Leena and is shot with a crossbow before she brutally murders him with his own fencing sword.
In a frantic finale, as the house ignites in flames due to their fight, Tricia and Leena find themselves clinging to the roof, pleading for Allen's help. With Leena deceitfully portraying herself as the victim, Tricia attempts to expose Leena's true nature. Torn between the two, Allen chooses to assist Leena, resulting in Tricia’s deadly fall from the roof. Upon realizing Leena's true identity after her false teeth fall out, Allen is horrified and calls her a monster. In a twisted act of rage, Leena retaliates by pushing Allen to his demise.
After these harrowing events, Leena once again disguises herself as Esther and escapes from the now-burning house, leaving behind a trail of chaos. Eventually, Leena finds herself in an orphanage, waiting for a new family to take her in, forever embodying the guise of Esther.
