Set in a seventeenth century Polish convent, the film begins as a priest, Father 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 a nearby convent after a local priest was burnt for sexually tempting the nuns. The next day, he sets out for the convent, where he meets its abbess, Mother Joan (Lucyna Winnicka), said to be the most possessed of all the nuns. Father Suryn will have to go to hell and back to save Joan, casting himself forever into darkness for her salvation.