As a child, Lucie Jurin escapes from a building where she has been held captive and tortured. She spends the rest of her childhood at St. Mary's Orphanage, where she is haunted by visions of a strange-looking creature that seems to attack her at every turn. Over time, she grows extremely close to Anna Assaoui, a fellow resident and Lucie's only friend.
Ten years later, Lucie travels to the countryside home of the Patterson family with a shotgun and executes all four members, convinced they are connected to her childhood torment. She calls Anna, who fears the family may not be responsible for Lucie’s torture. Anna is horrified by the scene when she arrives and calls 9-1-1 while Lucie sleeps, but she hangs up. She discovers Mrs. Patterson is still alive and tries to help her escape. Lucie suddenly tackles Mrs. Patterson outside and stabs her to death. While cleaning the crime scene, Anna finds a secret panel in a closet that leads to a hidden basement. There, she discovers a little girl named Sam and rescues her. The three girls regroup and safely make their exit from the basement, but trucks arrive with Eleanor, Fenton, and several other people who give chase; after a tense pursuit, all three girls are captured and taken into custody.
Later, Anna is interrogated by Eleanor, who explains that her group is a collective dedicated to discovering what waits in the afterlife. By torturing women and young girls to their breaking points, they believe they can create Martyrs with the ability to glimpse briefly into "the other side." Anna is thanked for bringing Lucie back and learns that Lucie is considered a rare find—someone capable of enduring great pain without dying. As an example, Eleanor has Anna watch from an overhead room as a captive woman is burned at the stake. After the woman dies, the priest overseeing the ordeal looks toward Eleanor and shakes his head. Lucie is taken for surgery, and the doctor performs a procedure that cuts and peels a piece of flesh from her back as part of the final preparations for martyrdom.
Meanwhile, Anna is buried alive outside for being Lucie’s accomplice, but she digs her way into a drainage ditch and escapes. She fights her way back into the underground facility, killing several cabal members. She frees Sam and tells her to run for help. While she searches for Lucie, she is attacked by Fenton and shot in the shoulder but ultimately kills him with the help of another captive woman. Anna is stabbed in the side by one of the cabal members before shooting him.
Anna finds and interrupts an assembly gathered to watch Lucie die on a cross. Lucie conveys a whispered truth to Anna, who insists that Lucie be freed. Lucie reveals what she saw in that moment, and Eleanor demands to know what was said; the priest, hearing the whispers, kills himself. Anna shoots Eleanor, and the remaining cabal members flee. Anna embraces Lucie as they lie dying on the floor. With the police on their way, both women’s eyes glaze over with the look of martyrdom.
