NOTE: This spoiler was submitted by Jeremy

Estonia, 2007 – A young woman named Anna (Gwendolyn Collins) arrives at the Saarne Institute for her first day at her job as the new art therapy director. As Anna meet with the head of the institute, Dr. Novotny (Dave Brown), the place goes into lockdown when it is discovered that their most violent patient, Leena Klamer (Isabelle Fuhrmann), has gotten loose. Anna encounters Leena in a dark room, falling for Leena’s ruse as a child patient. Before Leena can attack Anna, she is found and taken back into custody.

Dr. Novotny tells Anna about Leena’s condition, being that she has a gland disorder that makes her look like a child, and she has used this to her advantage in the past to get adopted into another family. She attempted to seduce the father, but it is implied Leena murdered the whole family, which got her sent there. She would fight against her restraints, leading her to have wounds around her neck and wrists.

An orderly named Dmitri goes by Leena’s cell. She convinces him to help her tie her ribbons around her by having him enter her cell, giving him the impression she will seduce him. Instead, Leena smashes Dmitri’s head against the wall until he is dead. She takes his coat and ID and starts to make her way out. A security guard stops her, but Leena gets another violent patient named Idti to attack and kill the guard with his own baton by offering Idti candy. Anna sees Leena escape on her way out, and she tells Dr. Novotny that the job is not for her.

Anna returns home and is bashed in the head by Leena. She then uses Anna’s computer to go through a missing child database and comes across a girl named Esther Albright, who Leena closely resembles. As she makes her plans, she sees that Anna is still not dead, so she hits her in the head a few more times. Leena then puts on clothing for a little girl and poses as Esther before an officer finds her.

In Darien, Connecticut, Tricia and Allen Albright (Julia Stiles and Rossif Sutherland) are watching their teenage son Gunnar (Matthew Finlan) in a fencing match. Allen has not moved on since Esther’s disappearance four years earlier. Detective Donnan (Hiro Kanagawa), who has worked on Esther’s case, approaches Allen and tells him there has been an update. Leena has come up with a fake story that she was abducted and taken to Russia to pose as a woman’s daughter until she found a way to escape. She is flown to the United States and “reunites” with the Albrights.

Tricia brings Leena to Dr. Segar (Samantha Walkes) for analysis. She slips up when mentioning a bird that is in Dr. Segar’s office, which the real Esther was familiar with. Leena turns on the PA speaker when Tricia is talking to Dr. Segar and overhears Tricia’s suspicions over “Esther’s” behavior, so she pretends that the bird attacked her and ripped her dress in order to get them to go home. On the way back, Leena sees Donnan taking pictures of her from a distance.

Back at home, Allen shows Leena his art studio that he abandoned after Esther had disappeared. She talks about how she knows how to paint, so Allen gives her charcoal materials for her to make a portrait of him. Tricia notices from afar the way that Leena is looking at the painting admirably.

During dinner, Donnan visits the family to check in on “Esther”. Leena grows nervous that he is onto her, so she hides and observes his conversation with the Albrights. Later, Allen and Tricia are preparing to get intimate, and Leena watches them. She then spitefully rips Tricia’s dress.

Allen and Tricia go to a gala for Tricia’s work, so Gunnar invites his friends over for a party. Leena comes by and attempts to join, but when Gunnar tells her to leave, she tells him to fuck himself. Soon, Donnan arrives and asks Gunnar some questions. He goes to use the bathroom and collects a sample from Leena’s room, which she notices. She then follows him home.

Tricia returns home and goes into Leena’s room and finds her journal from the Saarne Institute, as well as Donnan’s home address written down. Leena goes to his house just as he pieces together that she is not really Esther. She stabs Donnan in the back repeatedly and asks how he knew she is not Esther, to which he says that Tricia knows too. Donnan is then killed by multiple gunshots…fired by Tricia.

Tricia tells Leena she knows about her whole ruse, and she reveals why she is going to go along with it – the real Esther was killed by Gunnar in an accident, and rather than lose her other child by having him go to prison, she helped Gunnar cover it up. She helps Leena cover up Donnan’s murder by dumping his body down a hole (where it is implied Esther’s body is also down there) and sending an e-mail saying he is taking a break. For Allen’s sake, they continue the ruse because Tricia knows he will spiral if he finds out Esther is really gone. When they return home, Tricia tells Gunnar everything, and he is not happy that Leena has her grip on them.

Tricia schools Leena on how to convince everyone else that she is really Esther by telling her about memories that the real Esther would know about. Gunnar antagonizes Leena in private, but she slaps him twice.

Before dinner, Tricia tries putting a large dose of her medication in Leena’s mac and cheese to put her to sleep. Leena gets suspicious and takes the food to her room, where she places it near a vent where a rat she has noticed is staying. Allen goes to spend time with Leena and shows her blacklight effects in his paintings. Later, Leena sees that the rat has died from the tainted mac and cheese, cluing her in on Tricia’s intentions. Tricia also figures that Leena is attracted to Allen, but she taunts Leena and says he would never feel the same about her if he knew the truth.

Leena later prepares a breakfast for the family, but gives Tricia a smoothie with the dead rat inside. She sees it after drinking it and dumps the rat into the garbage disposal.

The family goes to a train station as Allen heads to work, where Leena makes a failed attempt to push Tricia and Gunnar onto the tracks. When Tricia drags Leena, she fights her way out and pepper-sprays Gunnar in the face before stealing Tricia’s car. Leena is found by police and brought back home.

Tricia and Gunnar then decide it is not worth having Leena pose as Esther anymore, so they try to kill her and stage it as a suicide. Leena breaks free and makes a run for it. Gunnar finds her alone in her room and he tries to attack her, but Leena stabs him to death. Tricia becomes enraged and goes after Leena just as the house catches fire. Leena and Tricia fight on the roof just as Allen gets home and watches them fall and hang on. Allen makes it in time to save Leena, but Tricia falls and smashes her head on the pavement. Leena’s fake teeth then fall out, making Allen realize her lies, but he ends up falling to his death as well before he can do anything about it. Leena continues to pretend to be Esther as authorities arrive.

Leena is taken in by Dr. Segar as she is prepared to be put into an adoption home, still believing she is sweet little Esther. Leena then appears to give a little smile to the camera before dropping it into a menacing scowl.

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Leena Klamer is a violent patient at the Saarne Institute who suffers from a gland disorder that gives her the appearance of a child despite being a 30-year-old woman. After she escapes, she poses as Esther Albright, a girl who had gone missing from her family four years earlier. Leena comes up with a convincing story and is taken in by the Albright family - Allen, Tricia, and their son Gunnar.

Leena is mostly good at fooling the family, especially Allen, but she gets nervous when Detective Donnan starts to piece together that she is not really Esther. When she goes to kill him, she is surprised to find Tricia showing up to finish the job. It turns out that Gunnar had killed the real Esther, and Tricia helped him cover it up so he wouldn't go to jail. They only choose to keep up the ruse for Allen's sake.

When Tricia notices that Leena is attracted to Allen, she and Gunnar start to antagonize her. After a failed attempt to kill them at the train station, Tricia and Gunnar attempt to kill Leena and stage it as a suicide, but she fights back. Leena stabs Gunnar to death and fights Tricia on the roof before Allen gets home. Tricia falls to her death, and Allen soon discovers Leena's lies, and he falls to his death as well.

Leena continues to pose as Esther and is put into an adoption home where she will be later adopted by the Coleman family.