Eli, a young boy afflicted by a rare condition that triggers severe allergic reactions to the outdoors, is forced to navigate life while clothed in protective gear. His parents, Rose and Paul, seek medical assistance at the isolated facility run by Dr. Isabella Horn, a large, vintage house that has been modernized and secured against the outside world. Initially filled with excitement, Eli feels liberated when he's allowed to remove his "bubble suit," embrace his parents, and finally enjoy simple comforts he has been denied for so long.
However, this newfound happiness quickly fades as Eli starts to encounter unsettling supernatural events throughout the house. His treatments become agonizingly painful, and he begins to hear ghostly whispers urging him to "lie." Eli's confusion deepens as he questions whether these spirits are warning him about the true nature of Horn's medical procedures.
Amidst this turmoil, Eli strikes up a friendship with Haley, a girl who he communicates with through a large window in the lower level of the house. Unlike others, she believes Eli’s claims that the house is indeed haunted, despite Horn's insistence to his parents that Eli’s experiences are merely hallucinations resulting from his treatment. Haley suggests that none of Horn's previous patients managed to leave the facility alive, intensifying Eli's fears.
With Haley's guidance, Eli deciphers that the ghostly message "lie" corresponds to the inverted number 317, leading him to the passcode for Horn's office. Inside, he discovers documents revealing that all previous patients perished during the critical third treatment. Desperate to escape, Eli attempts to convince his parents that they must leave, but his father deceives him into a false sense of security, ultimately aiming to sedate him.
Feeling betrayed and confused, Eli flees, barricading himself in Horn's office, where he uncovers a photograph of Horn and her aides dressed as nuns and a hidden entrance to an underground chamber adorned with a circular stone structure and religious artifacts. When Dr. Horn finds Eli there, she traps him inside, causing him to suffer an allergic episode and fall unconscious. Miraculously, when he wakes, he realizes he can breathe normally, revealing that his ailment was a fabrication all along.
Stricken by guilt, Eli's mother approaches him in the hidden chamber, but pretending to be unconscious, he surprises her and knocks her out using a heavy crucifix he finds hanging on the wall. Eli is soon recaptured by Dr. Horn and his father, who, after recovering, learns that the crucifix contains a dagger. Meanwhile, his mother uncovers the horrifying truth: the stone monument conceals the bodies of Horn's former patients, bound and exhibiting grotesque religious symbols.
Overwhelmed by rage, she confronts Horn upstairs, but her husband deceives her into relinquishing the knife. It is then shocking to discover that Eli is not afflicted by a disease at all; rather, he is a child of Satan, and his "allergic reactions" are manifestations of his developing powers. As Dr. Horn prepares for the final treatment, a deadly religious ritual, she splashes holy water on Eli, temporarily incapacitating him while she attempts to stab him with the sacrificial dagger.
In a dramatic turn, Eli taps into his latent telekinetic powers, lifting Horn and her accomplices off the ground and causing them to rotate before igniting them in flames. Their charred remains fall to the floor, igniting the house in an uncontrollable blaze. Eli’s mother, realizing she made a pact with darkness in her quest for a child, reveals that she was misled into believing Eli would be ordinary. As Eli’s father advances with the dagger, Eli unleashes his powers, resulting in his father’s devastating demise.
Eli and his mother escape the fiery wreckage, where they are reunited with Haley. In a surprising twist, Haley admits she is also a child of Satan and offers to guide Eli to his real father. When Eli accepts, Haley questions whether he can trust his mother. Eli reassures her that his trust remains intact, and the story concludes with his mother driving away alongside the two children, leaving the smoldering remains of the facility behind.
