Daniel Rodolfo Sancho and Sara Belén Fabra, who renovate and resell houses, move into a sprawling new home with their 8-year-old son Eric Lucas Blas. Eric is troubled by insomnia and hears voices that push him toward eerie crayon drawings. They bring in a psychologist Beatriz Arjona, who explains that the disturbances stem from a lack of stability, and expects him to settle down. Yet Eric produces a bloody branch, and while the psychologist drives home, a supernatural fly harasses her until she crashes into a tree and dies. The house is so large, and Eric's bedroom so distant from his parents' that they rely on walkie-talkies to stay in touch. Daniel discovers the home is infested with flies and struggles to eliminate them. Unexplained events begin to pile up: inexplicable stigmata-like scratches appear on their skin; a locked gate opens by itself; Eric's toy robot activates and speaks; and he believes his father expresses hatred for him through the walkie-talkie. One night, Daniel finds Eric drowned in the pool.
Sara leaves for a few days. The hauntings then focus on Daniel: he leaves a voice message for her, then hears Eric crying for help when he replays it, and the walkie-talkie crackles with static whenever he approaches it. He buys a book, Sounds from the Great Beyond (Sonidos de Más Allá), at a signing by parapsychologist Germán Domingo and asks for his help. Germán Ramón Barea and his daughter Ruth Ana Fernández arrive, Ruth armed with ghost-hunting gear—monitors, infrared cameras, EMF meters, headphones, and thermal imagers. Almost at once she orders Daniel out, but her equipment detects someone invisible in Eric’s room; Germán investigates, and Ruth pleads for him to leave, because there is a specter before him; a burst of noise in his headphones convinces him to join the others.
That night, Germán is awakened by the apparition of his dead wife, Sofia Nerea Barros; in the vision, she knives his arm, telling him he will join her. Ruth wakes him, and he discovers he has sleepwalked and cut himself. Meanwhile, Sara receives a phone call from Eric, who says he is hiding under his bed and begs her to help him against Germán, who wants to hurt him. She drives to the house and sees men burying the corpses of cats that had been found hanged in a tree. She goes to Eric’s bedroom, where she sees his ghost playing with his toy robot; he vanishes, and she encounters a corpse-like witch. Ruth arrives and tells them that the house sits on the grounds of a courthouse used for Spanish Inquisition trials. Sara’s body crashes through the window; she has hanged herself. Ruth tells Germán that they are leaving and begins packing, but he wants to stay to uncover the estate’s historical truth. In Eric’s room, Daniel sees the ghosts of his wife and son, then is attacked by an invisible malevolent force.
They realize that the evil is caused by the ghosts of those immured in the dungeon, the source of the flies, so Daniel breaks down the basement wall he had repaired. They discover a caged skeleton of a woman whom the Inquisitors used the pear of anguish on to prevent her from cursing. Daniel goes to fetch gasoline so they can burn the skeleton and free the demon. Ruth is visited by Sofia’s ghost but refuses to approach; Sofia takes on a demonic wraith form and possesses her. The ghosts of Eric and Sara beg Daniel to join them, and he pours gasoline over his head to immolate himself. Ruth stabs Germán, but he burns the skeleton just in time to free her. While she tends to him, he jokes that he’s looking forward to an exorcism his priest friend invited him to attend. Daniel visits Eric’s room one last time, then sees the disturbing drawings on the walls: they depict parts of the hauntings and deaths that afflicted them, including one that shows him possessed and drowning Eric and another of him shooting himself. Daniel takes his gun down to the pool and kills himself. A post-credits scene shows Germán and Ruth watching footage of the exorcism.
