Toledo, Spain, 1997. Sonia, an NGO leader, helps Moroccan strawberry pickers rebuild Fuensanta as their settlement. Radi, a Muslim boy, begins to show strange behavior after going into the ruins of the village church's basement. Safir, the religious leader of the Maghrebi community, believes it's due to the presence of a jinn, a Muslim demon.

Sahara Desert, March 1986. Safir, a soldier in the Moroccan army, is captured by the Polisario Front. Imprisoned in the desert, he meets Yusef, the spiritual leader of the prisoners. Yusef designates him as his successor, training him to become the new Imam and to face something unknown to Safir until then: the malevolent power of the jinns.

A piece of news deeply upsets Sonia. Safir explains that it may be connected to what dwells inside Radi. Something evil has emerged from Fuensanta's basement, and if it's out there, it must be stopped. To do that, they'll need help-someone of faith, true faith. Sonia believes that person could be Father Ángel, the Christian priest from Río Blanco.

The situation in Fuensanta is increasingly complicated. Ángel and Sonia, along with Omar and Fátima, are forced to perform ruqyah on Radi in the church of Río Blanco. Through the exorcism, Ángel discovers that something more than the evil caused by a jinn is hidden in those ruins.

Río Blanco, 11 years earlier. Toño, Carmen's father, locks and chains her inside the abandoned church of Fuensanta. From that day on, Carmen will never be the same, haunted by a voice in her head.

Safir, Sonia, and Ángel manage to descend to the foundations of Fuensanta's catacombs. There, they find something capable of unleashing their worst nightmares. The oldest pain. The most ancient fear. A primal darkness that has decided to emerge from the depths of Fuensanta.