The 'Nun of Monza' is the notorious story of Sister Virginia Maria de Leyva, first told in 'I Promessi Sposi' or 'The Betrothed' by Alessandro Manzoni in 1840. It is based in part on actual events that occurred in the 1630s in Spanish-occupied Italy, in which the Sister bore Gianpaolo Osio two children, and he committed several murders in a vain attempt to hide the truth. In this Bruno Mattei retelling of the story, Sister Virginia Maria (Zora Kerova) is sent to the convent of Monza, where she quickly becomes the target of the lecherous fugitive, Gianpaolo Osio, with the aid of his compatriot, the corrupt priest, Don Paolo Arrigone. But the pious Sister Virginia is also the Lady of Monza, mistress of the de Leyva estates upon the death of her father. Consequently, the Sisters elect her to be their Mother Superior when old Sister Imbersaga dies, and she selects the devious lesbian, Sister Benedetta (Paola Montenero), as her assistant. To protect his affair with Sister Candida, Father Arrigone blackmails Sister Benedetta into arranging a liaison between Virginia and Osio. The two are conveniently 'discovered' by Arrigone and Candida, and they allow Osio to rape Virginia as Benedetta watches and masturbates. Osio continues to press his desire for the now-compromised Sister Virginia, deluding her into thinking he truly cares for her, and that theirs is a spiritual union. Eventually Virginia turns up pregnant, while Osio is imprisoned for murdering a maid. The baby, however, is born dead, and an ambitious novitiate, Margherita and Osio (who is somehow no longer in prison) toss the body into the river, after which they make love. Sister Virginia repents of her sin, and commits Margherita to a penitence cell after she threatens the Mother Superior with exposure, supposedly to prepare herself for her vows of sisterhood. But Margherita doesnt relent in her threats against the convent, so the ruthless Osio is brought in to murder her. But its too late - the older nuns have called an inquisitor to the convent to purge it of the heretics. Osio escapes, but the three nuns, Virginia, Benedetta and Candida are imprisoned for life.