Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Initial Situation
Thérèse lives with her aunt Mme Raquin and Mme Raquin's sickly son, Camille. Of course, Camille and Thérèse get married. (Shall we call this unhappily ever after?) Thérèse spends most of her days staring off into space.
Thérèse has a highly passionate and nervous personality, but she feels oppressed by her marriage and her overall dull life at the haberdashery shop. She puts on a calm face all the time, but inside, she is bursting with energy (and sex-ergy) that's just begging to be released.
Conflict
Camille introduces Thérèse to his childhood friend, Laurent. Thérèse is immediately attracted to Laurent, and they quickly become lovers.
Thérèse and Camille don't seem to be attracted to each other at all. But Laurent unleashes Thérèse's sensual nature. Their affair is described as satisfying the two's animalistic needs—not their emotional needs. In the author's opinion (IAO?), there is no real love between Thérèse and Laurent.
Complication
One day, Laurent is prevented from making his usual sexual rendezvous with Thérèse. Deprived of their regular saucy outlet, Thérèse and Laurent are overcome by lust. So they begin to contemplate murdering Camille.
Notice the cause-and-effect chain here. The disruption of the lovers' routine is what leads them to thoughts of murder. Be on the lookout for how the author explores the causative relations between similar events and phenomena throughout the novel.
Climax
Laurent, Thérèse, and Camille go on a boat trip together. Laurent decides it's time to make his murderous move, so he pushes Camille overboard. Camille drowns, but Laurent and Thérèse are able to make the murder look like an accident.
The lovers have removed the obstacle preventing them from being together: Camille. So they are convinced that they'll soon be able to return to their old routine… of having lots and lots of sex with each other.
Suspense
Laurent and Thérèse eventually get married. But they haven't gotten it on since Camille's murder over a year ago. Meanwhile, Mme Raquin suffers a stroke, which leads to her discovering the truth behind her son's death.
Laurent and Thérèse are unable to reignite their passion for each other. They are haunted by Camille's ghost, and during one of their many arguments, they accidentally reveal their secret to Mme Raquin. But Mme Raquin's paralysis and loss of speech prevent her from denouncing the murderers. Tant pis.
Denouement
Laurent and Thérèse grow increasingly suspicious of each other. Mme Raquin watches and waits, hoping that the murderers will destroy each other. She's a sweet soul, ain't she? But you can't really blame her; she's seeking some kind of eye-for-an-eye deal for the murder of her son, Camille.
The hatred between Laurent and Thérèse is palpable now. They go out of their way to torture each other, which only drives them both deeper into madness and misery.
Conclusion
Laurent and Thérèse decide to murder each other. But when they discover their shared conspiracy, they are so horrified that they commit suicide together instead. Because that's just what you do. Oh, and Mme Raquin watches the whole thing go down, in her paralyzed silence.
Yes, this is quite the melodramatic conclusion to the book. Adultery + madness = double suicide, we guess. And let's not forget that Mme Raquin has gotten her revenge for her son's death.