Plot summary

Sister Mary sneaks into a superior's bedroom at night and steals a ring of keys to escape through the locked gates of the Roman Catholic convent. She is captured and knocked unconscious by four hooded figures. She later wakes up in a coffin and is buried alive.Sister Cecilia turned to Christianity at a young age after she nearly drowned in a frozen lake and was declared dead for seven minutes, convinced that God had saved her for a purpose. She receives an invitation from Father Sal Tedeschi to join a convent in Italy that tends to dying nuns in their last days. Cecilia takes vows to adhere to the evangelical counsels and becomes a nun, being decorated with a rosary.She befriends Sister Gwen and is struck by certain oddities, such as an elder nun having scars on her feet in the shape of crosses. The convent's chapel houses what they claim is a Holy Nail relic that was taken from the cross on which Jesus was crucified. Cecilia is shocked to learn that she is pregnant, even though she is a virgin and has never had relations with a man. The convent's inhabitants begin to treat her as the next Virgin Mary, with many proclaiming the child is a "blessing".Another nun, Sister Isabelle, tries to drown Cecilia out of jealousy, saying that it "should have been her." Cecilia throws up one of her own teeth and her health worsens. Her request to be taken to a proper hospital is denied by Father Tedeschi. Isabelle dies after falling off the convent's roof as Cecilia is walking through the cloister. Gwen publicly chastises the convent superiors for turning a blind eye but is then bundled away by Father Tedeschi and Deacon Enzo.At night, Cecilia notices that behind a painting of the Virgin Mary in her room, another nun had inscribed 2 Corinthians 11:14 on the wall, a hidden warning causing her to realize that something is wrong about this convent. Cecilia breaks open a drawer to read her file and is shocked to find information on the accident in her childhood. Hearing a woman screaming, she ventures further and is horrified to witness a hooded figure cutting out Gwen's tongue. An elder nun appears and silences Cecilia. She begs to escape but the elder chastises her, saying no one ever leaves the convent.She tries to escape the convent by faking a miscarriage using a dead chicken, but the deception is exposed. She attempts to run before being forcibly brought back by Tedeschi and Enzo. Tedeschi explains that he was a geneticist before becoming a priest and used DNA samples taken from the Holy Nail relic to impregnate nuns in the hopes of ushering in a new messiah. Until his success with Cecilia, Tedeschi's numerous attempts failed, often resulting in malformed fetuses. Tedeschi brands a cross into Cecilia's foot, a symbol also on many of the other nuns.Cecilia makes another attempt to escape. She beats Mother Superior to death using a crucifix, and shortly after, her water breaks. She manages to strangle the cardinal with her rosary before dousing Tedeschi's lab in ethanol. Tedeschi arrives and tries to stop her through physical force, but Cecilia narrowly escapes by igniting the ethanol with a lighter.Tedeschi manages to break out of the laboratory after extinguishing the flames and continues his pursuit against Cecilia, forcing her to flee into the catacombs under the convent. Cecilia finds Gwen's mutilated corpse along with a hole in the catacombs walls. Tedeschi finds her just as she is about to escape and a struggle ensues. Tedeschi tries to cut Cecilia's stomach open, but she stabs him in the throat with the Holy Nail relic, which she had previously stolen from the chapel. After exiting the catacombs, Cecilia agonizingly gives birth and bites through the umbilical cord herself. She is clearly horrified by the sight of the "baby", which is heard making gurgling and growling noises, while gasping for air. Cecilia stumbles away to grab and lift a nearby rock, which she uses to crush the experiment.