In a small French town during the Occupation, Barny (Riva)is a young, wayward, sexually frustrated widow, living with her little girl. She is also a communist militant who long ago decided that the easiest way was the best. One day she enters a church, randomly chooses a priest (Belmondo) to confess to and, while in confessional, attempts to provoke him by criticizing Catholicism. Instead of being affronted, the priest engages her in an intellectual discussion regarding religion. The priest is Leon Morin, young, handsome, smart and altruistic. He invites Barny to continue the conversation outside of confessional. She begins regularly seeing him and is impressed by his moral strength, while he makes it his mission to steer her onto the right path.
Source : Wikipedia