Set in a seventeenth century Polish convent, 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.
This film is very loosely based on the real life outbreak of mass hysteria in the French town of Loudun in 1634 that occurred when a convent of Ursuline nuns, led by the hunchbacked Sister Jeanne of the Angels, became obsessed with a handsome, womanising priest, Urbain Grandier. When Grandier turned down the nun's invitation to become their spiritual director, Jeanne, in a jealous rage, accused Grandier of using black magic to seduce her and her sisters and possess them with devils. Grandier's enemies, including Cardinal Richelieu, used the accusation as an excuse to have him found guilty of witchcraft and executed.
Unlike Ken Russell's The Devils (1971), which depicts Grandier's trial and death, Mother Joan of the Angels instead depicts the events after his death. The nuns continued to be possessed for four years after his death, and further exorcisms were carried out by the sincere and deeply spiritual Father Joseph Suryn whose main concern was helping Sister Jeanne.