The Duke of Sairmeuse returns from exile to claim possession of his lands, the majority of which are now in the possession of Lachneur, a bourgeois widower who live with his beautiful daughter, Marie-Anne. He claims that he had been charged with their guardianship until the return of the Sairmeuses, but the Duke treats him like a servant and accuses him of profiting from them. In their misfortune, one of their friends, the Baron of Escorval, under police surveillance as a former supporter of the Empire, asks Lacheneur for Marie-Anne's hand in marriage for his son, Maurice, who is in love with and loved by her. He refuses because he is planning an uprising against the Sairmeuses, and does not want Maurice to get caught up in it. Maurice becomes involved in the plans to be closer to Marie-Anne, joining Lacheneur's son, Jean, and Chalouineau, who is secretly in love with Marie-Anne. Stopping at nothing that could help him succeed, Lacheneur is even welcoming to Martial, the marquis de Sairmeuse, who is enamoured with Marie-Anne and hopes to make her his mistress. His fiancé, Blanche, the daughter of the marquis of Courtomieu, is furious and vows revenge on the woman she wrongly considers as her rival.
The uprising fails and the Baron d’Escorval is arrested as the head of the plot, despite having tried to dissuade the rebels from their course of action. He is condemned to death, along with Chalouineau, in a trial presided over by the Duke of Sairmeuse. The baron is saved by Chalouineau, who trades a compromising letter written by the Marquis de Sairmeuse for the chance for the baron to escape. The Duke and Courtomieu accept, but cut the cord that was to help the baron escape as soon as they get hold of the letter. The baron is badly wounded but carried away and cared for by the village curate, father Midon. Chalouineau is executed and leaves all his property to Marie-Anne.
Maurice and Marie-Anne reach Piémont, where a priest marries them in secret. They go to Turin, but Marie-Anne decides to return to France when she learns of her father's arrest and execution.
Maurice, unaware that Martial was not involved in the treachery against his father, writes a letter to him denouncing him. Martial, outraged by Courtomieu's bad faith, reads the letter at the wedding evening, causing a scandal. He vows to live apart from his wife. Marie-Anne takes possession of Chalouineau's house, and conceals the birth of her son, that a Piedmontese peasant takes away to his land in secret.
Blanche, still desiring vengeance against Marie-Anne, gets Chupin to spy on her and slips into her house when she is away and puts poison in a bowl of soup that Marie-Anne drinks on her return. She dies in agony, but sees Blanche who did not have a chance to escape. She pardons her on the condition that she takes care of the son she had with Maurice. Chupin is a witness, but later dies from a stab wound from one of his enemies, but not before revealing Blanche's crime to his oldest son. Martial vows to avenge Marie-Anne, but no-one suspects that Blanche is the murderer. They move to Paris and live separately under the same roof. They soon learn that the Duke is killed when he went out riding on his horse, probably by Jean Lacheneur, who is in hiding. Chupin's eldest son turns up in Paris and blackmails Blanche. She fails to find Marie-Anne's son.
Years pass, Maurice's parents die, and he becomes a judge in Paris. Chupin's eldest son dies, Blanche believes that she is free from the blackmail, but Jean Lacheneur arrives in Paris, aware of who killed his daughter, and decides to exact vengeance on her by using her husband. He makes Chupin's widow begin the blackmail again, and sends an anonymous letter to the duke to draw attention to her movements. Martial is stunned when he sees the seedy bar that his wife has been going to, but glimpses the truth when he finds out that it is owned by Chupin's widow. He finds a compromising letter that Blanche kept and realises that she murdered Marie-Anne.
Martial follows Blanche one night when she goes to the Poivrière to meet Chupin's widow with her chamber maid. Jean Lacheneur has set a trap, in which he intends to lead Martial and Blanche to a notorious place and provoke a scene in which they will find themselves compromised. However, the three criminals he enlists in this scheme let greed take over and try to steal Blanche's diamond earrings. Martial intervenes and has to combat three foes. He promises Chupin's widow a reward if she keeps quiet. The women manage to escape. This takes the reader to the beginning of the affair.
Having found out that Blanche has committed suicide and that M. d’Escorval has been reunited with his son, Lecoq decides to confront the Duke of Sairmeuse, having put together all the pieces of the mystery. One day a red-haired man goes to the Duke's house and gives him an urgent letter from M. d’Escorval, asking him, as a gesture of his gratitude for not revealing his identity, to lend him a large sum of money that he needs. Martial replies with a letter that tells him that his fortune and his life belong to his old enemy, whose generosity have saved him from dishonour. He hands this back to the messenger, who drops his beard and wig: it is Lecoq, who had forged M. d’Escorval's handwriting. The case against the Duke is dismissed, his innocence having been proven, and Lecoq is appointed in the post that he sought. (Source: Wikipedia)
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