The story takes place in and around a seventeenth century Polish convent. A priest, Father Józef Suryn (Mieczyslaw Voit), arrives at a small inn for a night's rest. He has been sent to investigate a case of demonic possession at the nearby convent after the local priest, Father Garniec, was burnt at the stake for sexually tempting the nuns. The next day, Father Suryn sets out for the convent, where he meets the abbess, Mother Joan (Lucyna Winnicka), said to be the most possessed of all the nuns. Already four priests before Father Suryn have tried to exorcise Mother Joan, but without success. The villagers at the inn are curious about the convent's troubled past and do everything to keep track of its developing story, with the stableman, Kaziuk (Jerzy Kaczmarek), leading Father Suryn around and asking the only non-possessed nun Sister Malgorzata (Anna Ciepielewska) for stories when she makes her nightly visits to the inn.
After Father Suryn learns that Mother Joan is possessed by eight demons, he and several other priests, during an exorcism, manage to exorcise the abbess. She and the other nuns appear cured. Soon after, however, the demonic possession increases. Mother Joan tries to seduce Father Suryn, begging him to make her a saint. In the meantime Sister Malgorzata leaves the convent and becomes Margareth after falling in love with Chrząszczewski (Stanisław Jasiukiewicz), a squire who visits the inn.
After a failed meeting takes place between Father Suryn and the local rabbi (also played by Voit), the priest re-enters the convent and receives Mother Joan's demons through his love for her. At night, reasoning that the only way to save the abbess is by doing Satan's bidding, Father Suryn grabs an axe and kills Kaziuk and Juraj, another stableman. The next morning, Margareth is abandoned by the squire, and finds Father Suryn holding the bloodied axe. The priest instructs her to go to Mother Joan and tell her of the sacrifice he made for her salvation in the name of love. Margareth runs back to the convent and cries with Mother Joan, neither saying a word.
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