In 1956, Father Noiret and Jacques perform their daily chores at a church in Tarascon, France. While investigating a disturbance, Noiret is raised into the air, set on fire, and burned to death, all the while Jacques watches as he holds onto the priest’s rosary.

Following the events at Saint Cartha’s monastery, Sister Irene now serves in a convent in Italy. Maurice works at a boarding school in France, where he has made friends with a young Irish girl named Sophie, as well as a teacher and her mother, Kate. Irene has a vision of Maurice asking her to save him, and the next day, she is asked by the Cardinal to investigate a series of deaths across Europe. While Irene travels to Tarascon, Sister Debra, a young novice, joins her without permission. During the train ride, Debra expresses her difficulty in accepting miracles, such as the Real presence of Christ in the Eucharist, while Irene explains to her the importance of faith.

At Tarascon, Irene has a vision where she follows Jacques into an alley but ends up getting choked by Valak, the demon who takes the form of a nun. The next morning, Debra tells Irene that she had met with Jacques and he had given her Father Noiret’s rosary. At the same time, in the boarding school, Sophie is bullied by her classmates and locked up in the abandoned chapel of the school where no-one is allowed to enter. The bullies tell her to look at a window with a goat on the mosaic, and claim that if she looks away, the devil will appear to haunt her. The sun shines through the mosaic and makes the eye of the goat shine red.

Irene and Debra travel to Palais des Papes and meet with a librarian who tells them that the demon they are investigating was an angel rejected by God and that the emblem on the rosary is the family crest of St. Lucy, who was martyred by a pagan. Though she was set on fire by the pagan, she miraculously did not die; St. Lucy’s family rescued her eyes and dispersed throughout the world. The librarian suggests that the demon is killing the descendants of St. Lucy because it wants this relic, the eyes, which were last known to be stored in a former monastery, which is revealed to be a boarding school in the present day.

Irene and Debra go to the school to find Maurice in distress with Sophie and Kate. Irene tells Maurice that the demon is inside of him, and is devastated by the revelation, but is then taken over by the demon and attacks Irene and Debra, before being knocked unconscious. Inside the chapel of the school, Sophie shows Irene, Debra, and Kate the game her classmates made her play and Debra uses a flashlight to make the eyes of the goat on the mosaic glow red. The red light points in the direction of where the eyes of St. Lucy are buried and Irene finds the relic.

Debra and Kate are separated from Irene and Sophie to protect the schoolgirls from a demon goat. Under the influence of the demon, Maurice escapes and attacks Irene, while Sophie grabs the relic and runs away. Maurice chases after Sophie and causes the bell tower building to collapse. Irene finds Sophie inside, and Debra reunites with the pair. Debra admits that she has seen a miracle and has faith. Irene uses the relic against Maurice until he manages to hold it himself. He lifts Irene in the air and sets her on fire, but she does not burn as she realizes that she is a descendant of St. Lucy and is able to harness the same power associated with the relic.

Irene and Debra begin to pray the Words of Institution used in Christian celebrations of the Eucharist and barrels of wine present at the former monastery become the blood of Christ. As the blood of Christ soaks Valak, the demon perishes and is damned back to hell.