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
The Vicomte de Valmont prepares for a double seduction. For his own enjoyment, he plans to seduce and destroy the Présidente de Tourvel, a married woman known for her piety and purity of heart. As a favor to his former lover the Marquise de Merteuil, and to take revenge against an enemy, he agrees to corrupt a young girl named Cécile who's engaged to be married to a man who had rejected the Marquise.
Act II
Valmont works his magic with Madame de Tourvel, at first in person and then through letters. He also pretends to help Cécile correspond romantically with her music teacher, the Chevalier Danceny. Once he's gained her trust, he rapes her. He also wins the heart of Madame de Tourvel.
Act III
The Marquise refuses to sleep with Valmont as a reward for his victory. They have a falling out that leads to Valmont's death, the death of Madame de Tourvel, the humiliation of Cécile and Danceny, and the exile of the Marquise from Paris society.