Plot
After performing an open heart surgery, Steven Murphy, a cardiothoracic surgeon in Cincinnati, goes to a diner where he meets 16-year-old Martin Lang, whose father died a few years earlier. Steven invites Martin to meet his family - comprising his wife Anna, their daughter Kim, and their son Bob. Martin makes a good impression on everyone, but seems to especially get along with Kim.
The following night, Steven goes to Martin's house for dinner at Martin's request. After Martin goes to bed, Martin's mother initiates sex with Steven; when he rejects her, she tells him that it is Martin's wish for them to have sex, after which Steven angrily leaves.
Martin visits Steven's office, complaining of chest pain. Steven runs tests on Martin, all of which produce normal results. Martin invites Steven over again, but Steven declines. Martin's demands for Steven's attention grow increasingly frequent and desperate, and Steven becomes gradually more frustrated and concerned, especially when he learns that Martin gave Kim a ride home on his motorcycle.
One morning, Bob finds himself unable to move his legs. Steven and Anna rush him to the hospital, where numerous tests are performed. Martin then reveals that his father died while Steven was operating on him. Martin blames Steven for his death and believes that to make things even, Steven must kill a member of his own family; if Steven refuses, all four will eventually die within a few days, beginning with paralysis, an inability to eat, and finally bleeding from the eyes. Steven has Martin ejected from the hospital. Later, Bob begins inexplicably refusing to eat, even when Steven begins furiously forcing him to.
Kim, who has fallen in love with Martin, collapses while singing in her school's choir and is also hospitalized. When she also starts rejecting food, she and Bob are both given feeding tubes. Steven finally confides in Anna about his history with Martin, eventually admitting that he had consumed alcohol before performing Martin's father's surgery. Martin phones Kim, instructing her to stand up and walk. She complies, only to become paralyzed again when he hangs up. Anna, witnessing this, confronts Martin and proclaims that he has no right to force her children to suffer. Martin asserts that this is merely justice for his father.
Kim and Bob are discharged. After Anna chastises Steven for his passive attitude toward their situation, Steven ties Martin up in their basement, beats him, and shoots his legs. Unintimidated, Martin bites Steven's arm, before biting his own arm as an example of retributive justice. Kim and Bob argue over which one of them Steven will kill, and each try to respectively curry his favor. Steven asks their school principal which of the two is better, though the principal cannot provide an answer.
Back home, Anna takes the children to see Martin, where she tends to his wounds and kisses his feet, ultimately failing to elicit any sympathy from him. She encourages Steven to kill one of the children rather than her, as they can have another child. Kim implores Martin to heal her so they can run away together; when this fails, she tells Steven how much she loves her family and offers her life to spare theirs. Hopeless, Anna finally releases Martin.
When Bob begins bleeding from his eyes, Steven places him, Kim, and Anna in a circle, bound with duct tape, and covers their heads. Steven pulls a wool hat over his own face and repeatedly fires a rifle while spinning in a circle. He narrowly misses Kim and Anna, but the third shot kills Bob.
Some time later, Martin enters the diner where he and Steven used to meet, where the remaining Murphys are. The three promptly leave, with Anna and Kim glaring at Martin, while Steven avoids his gaze.
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