The first third of the novel provides a lengthy exploration of the characters' histories. Balzac makes this clear after 150 pages: "Ici se termine, en quelque sorte, l'introduction de cette histoire." ("Here ends what is, in a way, the introduction to this story.") At the start of the novel, Adeline Hulot – wife of the successful Baron Hector Hulot – is being pressured into an affair by a wealthy perfumer named Célestin Crevel. His desire stems in part from an earlier contest in which the adulterous Baron Hulot had won the hand of the singer Josépha Mirah, also favored by Crevel. The Hulots' daughter, Hortense, has begun searching for a husband; their son Victorin is married to Crevel's daughter Celestine. Mme. Hulot resists Crevel's advances, and he turns his attention elsewhere.
Mme. Hulot's cousin, Bette (also called Lisbeth), harbors a deep but hidden resentment of her relatives' success. A peasant woman with none of the physical beauty of her cousin, Bette has rejected a series of marriage proposals from middle-class suitors, and remains unmarried at the age of 42. One day she comes upon a young unsuccessful Polish sculptor named Wenceslas Steinbock, attempting suicide in the tiny apartment upstairs from her own. As she nourishes him back to health, she develops a maternal fondness for him. She also befriends Valérie, the wife of a War Department clerk named Marneffe; the two women form a bond of mutual affection and protection.
Baron Hulot, meanwhile, is rejected by Josépha, who explains bluntly that she has chosen another man because of his larger fortune. Hulot's despair is quickly alleviated when he meets and falls in love with Valérie Marneffe. He showers her with gifts, and soon establishes a luxurious house for her and M. Marneffe, with whom he works at the War Department. These debts, compounded by the money he borrowed to lavish on Josépha, threaten the Hulot family's financial security. Panicked, he convinces his uncle Johann Fischer to quietly embezzle funds from a War Department outpost in Algiers. Hulot's woes are momentarily abated and Bette's happiness is shattered, when – at the end of the "introduction" – Hortense Hulot marries Wenceslas Steinbock.
Crushed at having lost Steinbock's company, Bette swears vengeance on the Hulot family. She works behind the scenes with Valérie to extract more money from Baron Hulot. Valérie also seduces Crevel and watches with delight as they vie for her attention. With Bette's help, Valérie turns to Steinbock and draws him into her bedroom. When Hortense learns of his infidelity, she leaves Steinbock and returns with their son to live with her mother Adeline. Valérie also proclaims her love to a Brazilian Baron named Henri Montès de Montéjanos, and swears devotion constantly to each of the five men.
Baron Hulot's brother, known as "le maréchal" ("the Marshal"), hires Bette as his housekeeper, and they develop a mild affection. He learns of his brother's infidelities (and the difficulties they have caused Adeline, who refuses to leave her husband), and promises to marry Bette if she will provide details. She agrees eagerly, delighted at the prospect of finally securing an enviable marriage. While investigating his brother's behavior, however, the Marshal discovers Baron Hulot's scheme in Algiers. He is overwhelmed by the disgrace, and his health deteriorates. Bette's last hope for a brighter future dies with him.
When Valérie becomes pregnant, she tells each of her lovers (and her husband) that he is the father. She gives birth to a stillborn child, however, and her husband dies soon thereafter. Hulot and Crevel are ecstatic when they hear this news, each believing that he will become her only love once the official mourning period has passed. Valérie chooses Crevel for his comfortable fortune, and they quickly wed. This news outrages Baron Montès, and he devises a plot to poison the newlyweds. Crevel and Valérie die slowly, their bodies devoured by an exotic Brazilian toxin.
Victorin Hulot is later visited by the Prince of Wissembourg, who delivers news of economic good fortune. The Marshal, prior to his death, had made arrangements for repayment of the Baron's debts, as well as employment for Adeline in a Catholic charity. Baron Hulot has disappeared, and Adeline spends her free time searching for him in houses of ill repute. She eventually finds him living with a fifteen-year-old courtesan, and begs him to return to the family. He agrees, but as he climbs into the carriage, Hulot asks: "mais pourrai-je emmener la petite?" ("But can I take the girl?") The Hulot home is reunited for a time, and Bette's fury at their apparent happiness hastens her death. One evening after the funeral, Adeline overhears Hulot seducing a kitchen maid named Agathe. On her deathbed, Adeline delivers her first rebuke to her husband: "[D]ans un moment, tu seras libre, et tu pourras faire une baronne Hulot." ("In a moment, you will be free, and you can make another Baroness Hulot.") Soon after burying his wife, Hulot marries Agathe.

Question: What deal does the Marshal make with Bette?
Answer: he will marry her if she gives him information about his brother's affairs
Alternative answer: Marshal promises to marry Bette if she provided detail of his brother infidelity.