1952 in Romania, two Nuns in an abbey are attacked by an unseen force when they enter a tunnel to receive a Christian relic. The nun who survives is known as Sister Victoria. She flees from the attacker, a demon appearing as a nun, and hangs herself. Frenchie discovers her body. He is a villager who transports supplies to the religious. The Vatican learns of the incident and sends Father Burke along with sister Irene to investigate the situation. The pair meets with Frenchie, who leads them to the abbey. They discover Victoria’s dead body and remove a key from around her neck. Inside, they encounter the Abbess, who informs them that the nuns experience a period of silence during the night and offers them lodging at the attached convent if they wish to live on to the next day. A demon attacks Frenchie as he returns to the village, luckily he gets away.

Burke tells Irene that a boy he exorcised, Daniel, was injured during the exorcism, and Burke is sad in the result of the boy’s death. Irene reveals that as a child, she had visions causing the Church to take an interest in her. Irene rescues Burke after being buried alive in the graveyard by the demonic entity. The next day Burk and Irene returned to the abbey, but only Irene can enter because it is cloistered. The nuns inside reveal to her that they have to pray every day to keep the entity at bay. One of the sisters discloses the history of the abbey. It was initially built as a castle by a duke who was obsessed with the occult. The duke summoned the demonic entity through a rift in the catacombs but was killed by Christian knights who then sealed the rift with an artifact containing the blood of Jesus Christ. The bombings during World War Two caused the rift to open again which unleashed the entity. Burke knows the entity as Valak from a book. He then discovers that the Abbess has been dead all along.

Irene gets attacked by Valak and attempts to join the nuns in desperate prayer to fend off the demon. She then realizes that none of the sisters are real and that she has been praying alone all along. Recognizing that Valak can only able to be stopped by sealing the rift with the blood of Jesus Christ, they retrieve the vial with the key Victoria had. Daniel’s ghost wounds burke as Valak starts to drown Irene in a flooded chamber. Irene splashes the blood of Christ in the demon’s face, banishing it as the blood of Christ seals the rift.