In 1957, a homeless, pregnant Mary, Sabrina Kern, flees her abusive father, Jayson Warner Smith, after the accidental death of her younger brother, Maximus Murrah, who plays William. With her boyfriend Jimmy on tour, she heads to a soup kitchen where a nun offers refuge in a convent, but the arrangement comes with hard rules enforced by the stern Mother Superior, Carolyn Hennesy.
Inside the convent, Mary is subjected to a tightening system of control. When she attempts to leave, she is drugged and renamed “Agatha,” and she is locked in a coffin that is opened only to be fed and to reinforce the nuns’ brainwashing. The atmosphere is claustrophobic, ritualistic, and designed to strip her autonomy away while presenting a facade of care.
During a donor breakfast, where the convent is judged by its patrons, Mary makes a daring escape and roams the woods until a bear trap injures her arm. The Wrights, a donor couple, retrieve her and bring her back to the house, where Mother Superior orders her wounded arm to be soaked in saltwater as punishment for her “rebellion.”
Jimmy tracks Mary down and visits the convent, but he is warned by Mother Superior that he will be killed if he remains. When Mary insists that she is now “Agatha” and that he is not the father of her unborn child, Jimmy leaves, broken and distraught.
In the same house, Sarah—a fellow resident—tells Mary that she believes the nuns took her healthy baby boy David at birth, lying that he had been stillborn. Mother Superior rebukes Sarah for speaking, then offers a grim path to safety: take a vow of silence and cut out her tongue to stay as a nun. Sarah agrees, sealing her vow.
Doris, another resident, goes into labor. Mary and Doris attempt a forced escape, but the pair are knocked unconscious by Mother Superior. Mary wakes later with unsettling dreams of her younger brother William and finds herself in the attic beside the corpse of her boyfriend, Jimmy. She threatens Mother Superior with a shard of glass to gain access to a telephone to call for help, but when a deputy arrives, the body has vanished.
Catherine, another resident, is killed after giving birth, and her child is handed over to the Wrights to be raised as their own. Mary confronts Mother Superior, who admits she was expelled from the Catholic Church but continues “God’s work” for the donors, profiting from the convent’s operation. Mary is offered a position as a nun, which she pretends to accept, but later returns to Mother Superior’s office to tear out a page of a book that reveals David’s whereabouts, passing it to Sarah.
In a bold move, Mary steals money to bribe three other nuns and the visiting doctor to aid her. The plan succeeds in locking Mother Superior away in a coffin. Sarah uses rat poison to incapacitate the remaining nuns, while Sister Paula, Trin Miller, chases Mary out of the house.
As Mary goes into labor, Sister Paula helps with the birth but then tries to seize the baby. Mary fights back, choking Paula with the umbilical cord and reclaiming her child. The final scene shifts to a sheriff’s operation outside the house, with officers preparing to open Mother Superior’s coffin. After the credits roll, Mother Superior’s voice is heard screaming, “Agatha!” adding a chilling coda to the tale.
