Cardillac has been murdered, and Olivier Brusson, Cardillac's assistant, has been arrested for the crime. Cardillac's daughter Madelon, who is betrothed to Olivier, protests his innocence. Because of Madelon's suffering and utter despair, Mademoiselle de Scudéri takes pity on her and takes her to her house to look after her. Touched by and believing Madelon's avowals of Olivier's innocence, the Mademoiselle tries to intercede on his behalf with La Régnie. He receives her graciously but is unmoved and presents her with circumstantial evidence that in his view proves that Olivier is the murderer. The Mademoiselle hears the evidence but cannot convince herself of the young man's guilt. La Régnie grants her permission to speak with Olivier, but when she meets him in prison she recognizes the young man who had thrown the warning letter into her coach and falls to the ground unconscious. She now is uncertain of Olivier's innocence and is torn inwardly. She curses the destiny that had made her believe in truth and virtue but now has destroyed the beautiful image she had made for her life. In the hope that Olivier will confess, Desgrais, de Scudéri's friend and an officer in the mounted police, offers to arrange for a meeting with Olivier in her house. The mademoiselle is filled with foreboding but nevertheless decides to obey the higher powers that had marked her for the solution of some terrible mystery. Olivier is brought to her house, and while guards wait outside he falls on his knees and tells her his story:  Olivier tells the mademoiselle that he is the son of the impoverished young woman, Anne, whom de Scudéri had lovingly raised as her own daughter and from whom she has not heard since she married an industrious and skilled young watchmaker who took her and Olivier to Geneva to seek their fortune. Because of the jealousy of others in his profession, Olivier relates, his father was not able to establish himself in Geneva, and both he and his wife later died there in poverty. Olivier, who had apprenticed himself to a goldsmith, eventually became so skilled in his profession that he was hired as an assistant by René Cardillac in Paris. All went well, Olivier tells the Mademoiselle, until Cardillac threw him out of the house because he and Cardillac's daughter, Madelon, had fallen in love.