Sister Mary secretly ventures into a superior's bedroom at night to steal a set of keys, aiming to escape from the locked gates of the Roman Catholic convent. Unfortunately, her plan backfires as she is captured and knocked unconscious by four mysterious hooded figures. When she regains consciousness, she finds herself in a coffin, buried alive.
Meanwhile, Sister Cecilia, portrayed by Sydney Sweeney, embraced Christianity at a young age after a harrowing near-drowning incident in a frozen lake, where she was declared dead for seven minutes. Believing that God spared her for a reason, she receives a troubling invitation from Álvaro Morte, who plays Father Sal Tedeschi, to join a convent in Italy that cares for dying nuns in their final days. Cecilia willingly takes her vows, adorned with a rosary, igniting her journey into the convent’s unsettling life.
As Cecilia forms a bond with Benedetta Porcaroli's Sister Gwen, she begins to notice unsettling peculiarities, including an elderly nun bearing scars shaped like crosses on her feet. The convent boasts a chapel that supposedly houses a relic known as the Holy Nail, claimed to have once belonged to the cross of Jesus Christ. The unsettling escalates when Cecilia discovers she is pregnant—despite her vow of virginity and having never engaged in any relations with a man. The other nuns quickly begin to regard her as the next Virgin Mary, often celebrating the child as a “blessing.”
However, the situation turns dire when Giulia Heathfield Di Renzi's Sister Isabelle, consumed by jealousy, attempts to drown Cecilia, insisting it “should have been her.” The turmoil within Cecilia accelerates; she becomes physically frail and starts to vomit her own teeth. Her pleas for medical assistance are callously dismissed by Father Tedeschi. Tragedy strikes when Isabelle falls to her death from the convent's rooftop while Cecilia quietly walks in the cloister, leading Betty Pedrazzi to publicly confront the superiors for their intimate blindness, only to be forcibly taken away by Father Tedeschi and Deacon Enzo.
In the depths of her growing paranoia, Cecilia discovers a hidden inscribed verse from 2 Corinthians 11:14 behind a painting of the Virgin Mary in her room—an ominous indication that something is fundamentally wrong within the convent. Desperate for answers, she breaks open a drawer and uncovers her childhood accident files. Her horror intensifies as she follows the screams and witnesses a cloaked figure brutally cutting out Gwen's tongue. In a desperate plea for freedom, Cecilia is silenced by an elderly nun's power, who coldly reveals the truth: no one ever leaves the convent.
Determined to escape, Cecilia devises a desperate ruse to fake a miscarriage using a dead chicken; however, her deception unravels, leading to her forceful capture by Tedeschi and Enzo. In a shocking revelation, Tedeschi, previously known for his work as a geneticist, confesses his ideations. He had been collecting DNA samples from the Holy Nail relic, testing them to create a new messiah by impregnating nuns. His failed attempts led to numerous malformed fetuses, marking him as a twisted figure of authority, branding a cross upon Cecilia’s foot that echoes throughout the convent.
Cecilia makes a courageous bid for freedom, resulting in her brutal confrontation with Mother Superior, whom she fatally wounds with a crucifix in a moment of sheer defiance. Following this, her water breaks, signaling the arrival of her child. In a moment of grotesque irony, she manages to strangle the Cardinal using her rosary before stirring chaos in Tedeschi’s lab, setting it ablaze with ethanol. As Tedeschi arrives, he is met with Cecilia’s fierce resistance, but she narrowly evades his grasp by igniting the flames.
As Tedeschi battles the flames to escape, he relentlessly pursues Cecilia, who seeks sanctuary in the dark catacombs beneath the convent. Amidst the shadows, she stumbles upon Gwen’s disfigured corpse and a hole offering a path to freedom. Just as Cecilia appears to slip away, Tedeschi emerges, sparking a fierce struggle. In an act of desperate survival, Cecilia plunges the Holy Nail relic into Tedeschi’s throat before she breaks free from the catacombs.
When she finally escapes, Cecilia undergoes the harrowing experience of childbirth, confronting the horror of her "baby," which emits unsettling gurgling and growling sounds as it gasps for air. In her bewilderment and terror, she takes a nearby rock and crushes the horrifying experiment, leaving a chilling imprint of survival intertwined with despair.
