Eli is a young boy suffering from a rare disease that causes severe allergic reactions to the outdoors, forcing him to live his life in protective gear. His parents, Rose and Paul, have taken him to Dr. Isabella Horn's secluded medical facility, in a large, old house that has been modernized and hermetically quarantined. Eli is initially overjoyed that the facility allows him to remove his "bubble suit", embrace his parents, and enjoy comforts previously denied to him. His joy is short-lived, however, as he begins to experience supernatural phenomena in various parts of the house. He also begins his treatments, which are excruciatingly painful. As the supernatural encounters increase, the specters repeatedly leave him the message lie, and Eli wonders if the spirits are trying to warn him about Horn's treatments.Eli befriends Haley, a young girl with whom he speaks through a large window in the house's lower level. She is the only person who believes his claims that the house is haunted, as Horn has told his parents Eli's visions are hallucinations caused by the treatments. Haley also tells him none of the other patients Horn treated left the facility, implying that they died. With Haley's help, Eli manages to discover the word "lie" is actually the inverted number 317, the passcode to Horn's office. When he investigates the office, Eli finds Horn's records of past patients, showing that all of them were killed by the third, final treatment.Eli unsuccessfully tries to persuade his parents that the three of them should leave the facility. His father tricks him into a false state of security, then tries to drug him into unconsciousness. Hurt and confused, Eli runs away and barricades himself in Horn's office. He finds a photograph of Horn and her assistants dressed as nuns. He also finds a hidden passageway to an underground room with a circular stone monument and religious paraphernalia. When Horn discovers him there, she locks him in, and he experiences an allergic reaction and passes out. When he awakes, he finds he can breathe fine, and his disease was a lie. His mother, feeling guilty for deceiving Eli, comes to Eli in the hidden room, where he pretends he's still unconscious. When she opens the gate and approaches him, he knocks her unconscious with a heavy crucifix he has removed from the wall.Eli is recaptured by Horn and his father. Meanwhile, his mother regains consciousness and finds that the crucifix with which Eli struck her contains a dagger. She then discovers that the stone monument conceals the bodies of Horn's previous patients, bound and adorned with large religious symbols. Horrified and angry, she goes upstairs and uses the dagger to force her way into the treatment room. However, her husband tricks her into turning the knife over to him.It is then revealed that, rather than having a "disease", Eli is a child of Satan, and that his "allergic reactions" were manifestations of his emerging powers. Horn begins the third "treatment"-a religious ritual meant to end Eli's life. Horn throws holy water on Eli, temporarily disabling him, and tries to stab him with the sacrificial dagger. However, Eli summons his powers and uses telekinesis to lift Horn and her two assistants in the air. Eli causes them to spin around him, upside-down, then burst into flame. Their bodies drop to the floor and set the house afire. Eli's mother reveals that she wanted a son so badly, she turned to Satan, who lied that Eli would be a normal child. Eli's father advances with the sacrificial dagger, but Eli immobilizes him and kills him by crushing his face.Eli and his mother leave the burning house, where Haley greets them. She reveals that she too is a child of Satan, and that she was unable to save him, as he had to find and prove his own power. She offers to take Eli to his true father. When he accepts, Haley wonders if he can trust his mother. He indicates that he can, and the film ends with the mother driving the two children away from the burning facility.