In 1952 Romania, two Catholic nuns living at Saint Cartha's monastery are attacked by an unseen, malevolent presence behind an ancient door marked with Latin warnings. The surviving nun, Sister Victoria, hangs herself from an upper balcony window holding a strange ornate key. Days later, a French-Canadian villager nicknamed Frenchie discovers her rotting corpse while delivering regular food supplies to the cloister.
Alerted to the suicide in a consecrated abbey, the Vatican orders Father Burke, an exorcist burdened by past failures, and Sister Irene, a novitiate experiencing clairvoyant visions, to investigate the spiritual integrity of the grounds. Guided by Frenchie, the pair travels into the mountains to reach the isolated, mist-shrouded fortress. Upon arrival, they meet the veiled Abbess, who insists they must wait until the following morning to interview the cloistered nuns, offering them lodging in the convent's guest rooms.
That night, evil strikes immediately. Father Burke is lured out into the fog by the specter of Daniel, a boy he failed during an exorcism years prior. Burke is pushed into an open grave and buried alive in a bell-rigged coffin. Alerted by the ringing cemetery bells, Sister Irene rushes to the graveyard and excavates Burke just as decaying hands reach for him inside the dirt.
Returning inside the catacombs, Irene meets Sister Oana, who explains the dark history of the castle. Built in the Middle Ages by a sadistic Duke, the fortress was used for occult rituals that tore open a rift to hell, summoning the demon Valak. Crusader knights breached the castle, sealed the rift with a relic containing the Blood of Jesus Christ, and nuns maintained perpetual prayer ever since to keep the portal shut. However, bombing runs during World War II cracked the seal, releasing Valak back into the halls.
Irene joins the remaining nuns in desperate, round-the-clock prayer in the chapel, only to realize a horrific truth: every nun she interacted with has been dead for days. The entire convent was slaughtered before they even arrived, and Valak used their spectral illusions to manipulate Irene into entering the crypts. Valak needs a living, unvowed vessel to possess so it can cross the consecrated threshold and escape into the world.
Burke, Irene, and Frenchie reassemble to fight through hordes of faceless, veiled entities. Recognizing the danger, Irene takes her solemn vows to become a fully consecrated bride of Christ, closing the vessel loophole. Arming themselves with the relic vial of holy blood, they descend into the flooded catacombs where the hellmouth lies.
