The novel is set around the year 1820. Auguste de Maulincour, a young cavalry officer, walking in a Parisian district of ill repute, sees from afar a young married woman, Clemence, with whom he is secretly in love. He sees her enter a house of ill repute. The young married woman is widely recognized in Parisian society as a paragon of marital virtue—what is her secret? Auguste decides to spy on the woman to find out. He finds her in the company of a man named Ferragus. In the days that follow, Auguste uncovers the secrets of powerful and mysterious people and escapes several assassination attempts. Subsequently, Auguste is killed, but before he dies, he reveals Clemence's secret to her husband, Jules Desmarets, a very rich stockbroker. Ferragus is none other than Clemence's father. It is revealed that Ferragus used to be a rich, handsome boy, a member of the Order of Devorants, who was imprisoned in 1806. He had escaped imprisonment and lived in Paris under various names and disguises. In 1815, Ferragus became involved in several sordid businesses.