Set in 1952 at the Carta Monastery in Romania, the unsettling tale begins with two nuns making their way down a shadowy hallway toward a door ominously marked "God ends here." The elder nun ventures through the door in search of a mysterious artifact, while the younger nun, [Sister Victoria](charlotte-hope), portrayed by Charlotte Hope, envelops herself in prayer. The atmosphere quickly shifts when the elder nun returns grievously injured, urging Victoria to flee before she is pulled back into the darkness. As dread fills her heart, Victoria, armed only with a key, is confronted by the sinister demon nun, [Valak](bonnie-aarons), embodied by Bonnie Aarons, and makes the heart-wrenching decision to take her own life.
Some time afterward, her decayed body is discovered by [Frenchie](jonas-bloquet), a character played by Jonas Bloquet. The Vatican learns of Victoria's tragic end, categorizing it as an ultimate sin that demands further investigation. They summon [Father Anthony Burke](demian-bichir) (played by Demián Bichir) to delve into this mystery, enlisting the help of a novice nun, [Sister Irene](taissa-farmiga), represented by Taissa Farmiga, who currently teaches young girls. The three of them set out to uncover the dark secrets of the abbey where Victoria was found.
Upon arriving at the abbey, they discover troubling signs—Frenchie finds Victoria's body positioned in a way that raises alarm, and a key clutched in her fingers. They give her a proper burial but face numerous obstacles, including a persistent feeling of dread. During their search, Burke recalls a haunting incident involving a possessed boy named [Daniel](august-maturo) (played by August Maturo) that continues to burden him with guilt.
As the night unfolds, Burke has a harrowing experience that leads him to a coffin while Irene grapples with her own visions of Valak. These visions unearth a troubling connection between their current plight and the tragic past of the abbey. [Sister Oana](ingrid-bisu) (portrayed by Ingrid Bisu) elucidates Valak's origins, revealing that the demon was summoned during the Dark Ages by a duke with malicious intentions, but was temporarily trapped due to the actions of others.
In a tense moment, the three protagonists confront the dark forces that lurk within the abbey, only to realize that Victoria’s sacrifice was meant to thwart Valak from claiming her body. They venture into the catacombs, guided by the phrase "Mary points the way," ultimately leading them to the critical artifact containing the blood of Christ. However, as they gear up for their final confrontation with Valak, tensions rise. Burke reads about the demon’s true name, adding another layer of complexity to their mission.
In an action-packed climax, Valak attempts to overpower Irene, while Burke faces the embodiment of his past fears. With quick thinking and resolve, the trio perseveres, using the blood to repel Valak and seal the rift that unleashed such terror.
As the dust settles and they emerge from the shadows of the abbey, Irene discovers Frenchie's true name, Maurice, and an unsettling marking on his neck hints at dark consequences yet to unfold. The film draws to a close by connecting back to the present through a scene from the first "Conjuring" film, where [Carolyn Perron](lili-taylor) (played by Lili Taylor) attends a seminar led by [Ed](patrick-wilson) and [Lorraine Warren](vera-farmiga) (portrayed by Patrick Wilson and Vera Farmiga). Their intertwined fates echo the broader saga of Valak, indicating that the horrors are far from over.
