Thérèse has poisoned her husband, but has thankfully been acquitted, thanks in part to a testimony from her victim. What brought her to poison him? Will he forgive her? She has time to think about this on the train ride back to their countryside estate, reflecting on their relationship, her motivations, and what apology she might be able to offer him.
Like much of Mauriac's work, the psychology of the characters is central. A contemporary telling of this story would likely try and find concrete, solid motivations for Thérèse's actions. Here, bravely, Mauriac opts to keep the motivations vague; in doing so, he portrays the reality of crimes like these more accurately.
Thérèse herself doesn't even quite know why she poisoned him. She has grievances, and the structure and expectations placed on her, with marriage as it is at that time, are certainly oppressive; she feels these pressures, but they aren't enough to give her a conclusive reason to act in this way. It's not as if there's a concrete motivation, like her refusing to let her divorce him, or something that specific.
Another dynamic is portrayed between a friend of Thérèse's, her husband's half-sister, Anne, and a lover she takes on, despite being in a nunnery. (I'm not sure if this is quite the right equivalent of "couvent", which is more like an educational boarding school with the potential of becoming a nun later.) She's being played by this guy, and so Thérèse needs to intervene to get him to leave her alone, to let her marry the "correct" choice for the reputation of the family.
This plot is important, in that it emphasizes two key feelings that motivate Thérèse: her feeling of being trapped in this relationship, and her tenderness for Anne, which contrasts with the disgust she feels for her own husband. There are some hints---merely that, really---of romantic feelings towards Anne. I would describe this more as an admiration, a tenderness. For her husband she does not feel such things, which is a partial explanation of her actions.
Without spoiling the evolution of the latter half, the author makes the interesting choices in how he has the characters act, which makes them seem much more fully-fleshed. Further layers of their psyche are demonstrated through their actions here. Strengths are revealed.
The novel does some interesting things with form as well. It's still a straightforward narrative, but there are some interesting plays with tense, switching from the past to the present tense, to create a kind of focusing of our point of view. There's also some cinematic elements. The initial chapters of the book are quite interesting through this lens, where the author splices together the present tense and imagery of the train ride home with the past tense of Thérèse remembering and trying to construct an explanation for her actions. Very interesting narrative techniques, although the latter half of the novel plays our more straightforwardly.
In summary, worth reading for both the narrative form, and the subtle psychology.