For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Thérèse has always been bored with her simple life, which basically consists of Camille, Mme Raquin, and the habadashery shop. So when she meets Laurent, her pent-up passion is unleashed—rawr—and she eagerly enters a life of adultery.
When circumstances prevent Thérèse and Laurent from meeting up for sexytime, the two conspire to murder Camille. And then they follow through on their plot. While on a boat trip, Laurent drowns Camille.
According to the outside world, however, Camille's death was an accident… Thérèse and Laurent are shifty characters indeed.
Act II
Laurent and Thérèse realize that, after freeing themselves of Camille, they're not attracted to one another anymore. They even get married in an attempt to save their relationship. But they're still unable to rekindle the intensity of their first months together.
Then Mme Raquin suffers a stroke, and Laurent and Thérèse accidently reveal their secret to her. In a cruel twist of fate, since Mme Raquin can't speak, she is unable to tell anyone about the crime these two perpetrated against her son.
Act III
Laurent and Thérèse spiral more and more out of control. They eventually grow so suspicious of each other that they each decide to kill the other. Laurent gets hold of some poison, and Thérèse sharpens a kitchen knife.
Right before they're about to go through with their schemes, however, Laurent sees the knife and Thérèse notices the poison. They're both so shocked by their own lies that they decide to commit suicide together. (Which is one way of "solving" their problems, we guess.)
Mme Raquin has been watching this whole scene unfold. The novel ends as she feasts her eyes on the dead bodies of her son's murderers. Eep. Mme Raquin just might be the scariest character in this book.