3 Rigor Mortis

In 1952 Romania, two nuns living at the Saint Cartha's monastery are attacked by an unseen evil force after entering a tunnel to retrieve an ancient Christian relic. The surviving nun, Sister Victoria, flees from the attacker, a demon appearing as a nun, and hangs herself. Her body is discovered by Frenchie, a villager who transports supplies to the nuns.

The Vatican learns of the incident and summons Father Burke to Rome, where they ask him to travel with Sister Irene, a nun in her novitiate, to Romania in order to investigate the situation. While Sister Irene is teaching children the relationship between religion and science in a school, her Mother Superior interrupts her and informs her that Burke has arrived in order to request Irene's accompaniment in his trip to Romania.

The pair travels to Romania and meets with Frenchie, who leads them to the abbey. They discover Victoria's body and take a key from her corpse. Inside, they encounter the Abbess, who informs them that the nuns observe a period of silence during the night and offers them lodging at the attached convent if they wish to return tomorrow. Frenchie is attacked by a demon as he returns to the village, but escapes. Burke tells Irene that a boy he exorcised in the past, Daniel, was fatally injured during the ritual, and Burke has carried the burden of the boy's death with him ever since. Irene reveals that as a girl, she had visions and each of them concluded with "Mary points the way", causing the Church to take an interest in her. That night, Burke is rescued by Irene after being buried alive in the graveyard by the demonic entity.

The next day, Irene and Burke return to the abbey, but only Irene can enter as it is cloistered. She meets some of the other nuns and learns that they are praying constantly, swapping in shifts, to keep the evil entity at bay. Sister Oana reveals the abbey's history: it was built in the Middle Ages as a castle by a duke obsessed with the occult. The duke summoned the demon through a rift in the catacombs but was killed by Christian knights, who sealed the rift with a vial filled with the Blood of Christ. However, the bombings during World War II accidentally reopened the rift, unleashing the evil entity once more. Burke identifies the demon as Valak and discovers the Abbess has been dead all along.

Frenchie heads back to the abbey to help Irene and Burke. Irene is attacked by Valak and joins the nuns in desperate prayer to ward off the demon. When the group reunites, Irene discovers that none of the nuns she had seen and talked to were real and she had been praying alone, later realizing that Victoria had been the last nun in the abbey and had sacrificed herself to stop Valak from possessing her body and unleashing evil.

Theorizing that Valak can only be stopped if they seal the rift with the blood of Christ contained in the reliquary, the trio retrieves the vial with the key Victoria possessed. Irene then informs Burke that God has called her to make her solemn vows as a nun and asks Burke to elevate her to the status of a professed nun, which he does in the abbey's chapel.