The novel is set in World War II and part of it concerns the Nazi occupation, the hiding of Jews, the work of the maquisards, and inevitably the collabos. Barny (the narrator) is a communist and atheist widow of a Jew who is now bringing her daughter France up on her own. But this is not really the main source of the drama, which lies between Barny and the young priest Léon Morin.
Barny one day just walks into church and starts talking to Morin, who in some ways thinks she's more religious than many of his flock who merely go through the motions. He lends her religious books which she reads and finds herself being converted to Catholicism, and their intellectual conversations continue through to after the end of the war. Morin is a fervent Catholic, which is clear from the beginning, and it is evident that nothing will sway him from his path. But Barny takes some time to realise that this man is not only intellectually engaging, but physically appealing. Although Zola's La Faute de l'abbé Mouret (The Sin of Abbé Mouret) this isn't.
I was occasionally reminded of Kazuo Ishiguro's The Remains of the Day (1989), with the lack of sexual content being foregrounded for that very reason. And indeed most of the things that need to be said here remain unsaid. However, Morin can say some strange things at times, and on seeing her with sandals and unvarnished toe-nails he seems surprised, and says Barny needs a man. She says she uses a piece of wood, although this doesn't seem to surprise him, and he says she ought to be careful, but Barny says she's tough.
After the war Morin finds it more convenient to see Barny and France at their home, cue for a neighbour to sing a slightly saucy song about a curé getting a young woman pregnant, which Barny finds 'obscene', but then this is the late 1940s. But oh, that sexual tension. France wishes Morin were her father, and certainly he has very fatherly features towards her.
But of course this can never be, even though Morin has told Barny (by the proxy of France) that he loves her. When Barny asks him if, had he been a Protestant vicar, therefore able to marry her, he would have. Yes, he would have. But there's a dazzling piece (shortly after Barny has had a dream about Morin coming into her bedroom and making immensely satisfying love to her) in which Morin visits Barny and she nearly loses control and Morin has to calm her (again, obliquely). He advises her to go to confession that day, which she does and tells Morin that she tried to seduce a priest.
So there we have it. Morin is moved to a completely different area, where the rural parishioners have not been spiritually guided for years, and Barny chooses not to ask Morin if it's by his choice that he's moving, or because he's genuinely been moved. Although in either case, he's obviously been severely emotionally moved. The novel would of course have been a disastrous, staggering cop-out if they'd, er, come together at the end.