It was adapted by Melville from Béatrix Beck's novel The Passionate Heart (French: Léon Morin, prêtre), which won the Prix Goncourt in 1952.

Set during WWII in Occupied France, the film stars Emmanuelle Riva as a jaded, lapsed Catholic mother and widow of a Jewish husband, who finds herself falling in love with a young, altruistic priest, played by Jean-Paul Belmondo.

She decides to talk to Léon Morin, who, being a young curate with a name that does not strike her as bourgeois, seems most likely to take the joke in good humor.

In the confessional, she attempts to provoke Léon by questioning and criticizing Catholicism, but, instead of getting offended, he engages her in a calm theological discussion.

She begins to visit Léon regularly, finding his moral strength and the steadfastness of his faith impressive as he works to help her on her spiritual journey.

The other women's reactions to him cause Barny to see him in a different light, and, noticing for the first time how handsome he is, she begins to develop romantic feelings.

They catch France eavesdropping and he puts her back to bed, prompting Barny to think about God's irony in finally bringing Léon into her bedroom, not to sleep with her and break his vows, but to comfort her daughter.

The correspondence school announces it is moving back to Paris, which means Barny will be leaving the town around the same time as Léon.