Sister Cristina is lost in the catacombs under the convent while searching for Sister Assunta. She finds her in what appears to be a laboratory in the convent's morgue. While Sister Assunta is preparing a nun's corpse for embalming, she relays the dangers the nuns face in temptation by Satan with lustful desires; she also tells a story of a child born dead in the convent to a nun who fornicated with the devil, and events surrounding the murder of the previous mother superior, Sister Florence. She claims Sister Florence's ghost still haunts the convent, calling the name of the nun who murdered her. Then, as if under a supernatural influence, she horribly murders Sister Cristina before dying herself. Mother Vincenza discovers the corpses of both nuns and justifies the deaths as accidental to the investigating Father Inardo. However, Sister Rosaria shouts that the devil lodges in the convent and is rebuked by Mother Vincenza. Sister Rosaria bleeds from her mouth when Father Inardo administers communion; afterwards, locked in her room, she experiences the symptoms of stigmata. Father Inardo is purifying the convent with the Mother Superior and the rest of the nuns in tow, until they are alerted by the cries of pain of Sister Rosaria. They rush to her room to find the nun has been horribly murdered, the bed and walls covered in blood.
Father Inardo and the Bishop, in order to unravel the situation, charge young Father Valerio with the investigation. He soon arrives at the convent where he discovers, in recent times, other supernatural events seem to have occurred in addition to the death of the previous nuns. Father Valerio understands that the whole environment is dominated by the dictatorial figure of Mother Vincenza and frequently clashes with her during the investigation, coming to suspect she is hiding something. After various vicissitudes, including the deaths of Father Inardo and the gardener Boris, Father Valerio discovers that the basis of the bloody facts is Elisa, the illegitimate daughter of Mother Vincenza. She has been kept segregated from her mother in a hidden room of the convent, her face horribly disfigured. A flashback reveals that soon as she was born, in fact, she had been thrown into boiling water by the previous mother abbess, Sister Florence, in order to dispose of a child born of a clandestine relationship between Sister Vincenza and a stranger. At the time of the immersion, the little girl, however, had managed to drive Mother Florence away with the power of her mind and caused her to strangle herself. Elisa, now a teenager, is endowed with supernatural powers, and is exploited by Mother Vincenza, in order to eliminate people who became aware of her motherhood or disobey her strict orders. Elisa had caused, by the strength of her thought, all the deaths in the convent.
Having discovered the horrible truth, Father Valerio is attacked and brutally stabbed by an enraged Mother Vincenza. Mother Vincenza reveals that she made a pact with the devil and renounced Christianity, and that Elisa is the daughter of Satan, thus confirming Sister Assunta's story from the beginning. Elisa attempts to save Valerio but Mother Vincenza strikes her from behind trying to kill her. A furious struggle ensues between mother and daughter, with Elisa chasing Mother Vincenza through the convent and the catacombs into the morgue. Now cornered, Mother Vincenza begs forgiveness of her daughter but she fatally wounds her with a dagger when Elisa stops the attack. Before dying however, Elisa manages to kill her mother, reviving a corpse to strangle the former nun. Later, after the deaths of Elisa and Mother Vincenza, Father Valerio is hospitalized, having lost his senses. The ecclesiastical authorities inspecting the convent find the laboratory in the morgue where Mother Vincenza practiced black magic. While the bishop and the new abbess are investigating, some hellish forces still seem to haunt the laboratory. They attribute the facts to some sudden earthquake, but as soon as calm returns, the corpse of a nun falls from a coffin with a terrible cry, landing in the arms of the bishop.
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