The Santa Cruz dry-goods store in Madrid, established in the eighteenth century, provides an income for Juanito Santa Cruz. Graduating from the university at the age of twenty-four, he is not yet ready to take his place in the family business. He wants to enjoy life, and Barbara Santa Cruz, his mother, spoils him. Her chief adviser is a former clerk, Placido Estupiñá, who smuggles goods into the city in his spare time.

At the home of a fellow student, Juanito meets the attractive Fortunata and takes her as his mistress. Shortly afterward, Estupiñá finds out about the affair, and Juanito’s mother contracts for him a marriage with his beautiful but passive cousin, Jacinta. They are married in May, 1871. When they return from their honeymoon, Fortunata leaves Madrid.

Jacinta discovers in the passing years that she cannot have children. Learning some details of her husband’s earlier affair with Fortunata, including the fact that his mistress bore him a son nicknamed Petusin, she wonders whether it is her duty to take care of the child. In the meantime, Juanito is told that Fortunata is back in Madrid. He immediately begins to look for her, but his search ends when a lung infection disables him for a long time.

Among Fortunata’s admirers is the ill-favored and schizophrenic Maximiliano Rubín, the orphan of a goldsmith, who, like his two brothers, is subject to violent headaches. Thin and weak, he was reared by his Aunt Lupe, who allowed him to live in a world of his own imagination. While studying to become a pharmacist, he meets Fortunata at a friend’s house. Her poverty affords her the opportunity to overlook his ugliness and to date him. When she confesses her past, he proposes marriage in order to redeem her.

Hearing of his plan, Aunt Lupe sends one of his brothers, a priest, to talk to Fortunata. The woman says frankly that Maximiliano is the only one of her lovers—except one now married—for whom she ever cared. The priest proposes that she spend some time in a home for wayward girls; if she benefits by the experience, he will agree to the marriage. After a term in the institution, Fortunata marries Maximiliano on a day when he is suffering from one of his worst headaches.

Knowing beforehand of the proposed marriage, Juanito takes a room in the boardinghouse that Fortunata and her husband are to occupy. At first he intendeds only to see Fortunata again, but on the night of the wedding her husband is ill, and Fortunata and Juanito resume their old intimacy. Maximiliano, finding out about the affair, quarrels with Juanito, who overpowers the puny pharmacist and sends him to the hospital with an injured larynx. Then Fortunata packs her belongings and leaves her husband.

Juan Pablo, the second of Maximiliano’s brothers, spends his afternoons in one café or another with his cronies, among them the elderly Colonel Evaristo Feijóo. While watching the parade marking the restoration of the monarchy in 1874, one of the loiterers sees Juanito and Fortunata sharing a balcony. Through gossip, Jacinta learns of her husband’s infidelity. When she accuses him, he arouses her sympathy for Fortunata by telling how badly she was treated by her husband. Nevertheless, he does promise to break off relations with the woman. His farewell message, with an enclosure of one thousand pesetas, so angers Fortunata that she goes to his house in order to create a scandal. The sight of Jacinta’s gentle beauty tempers her anger, however, and while she is trying to decide what to do she sees Colonel Feijóo. He points out that, untrained as she is for any career, she has only three choices: go back to her husband, accept the attentions of any man with money to pay her, or take him as her protector.

She chooses Feijóo as her lover, at the same time planning to make her future secure after his death and to reinstate herself in the good graces of the Rubín family. On one occasion, Fortunata comes face-to-face with Jacinta, who does not know what her husband’s former mistress looks like. Torn between a realization of Jacinta’s beauty and goodness and her hatred for her as Juanito’s wife, Fortunata blurts out her identity, much to Jacinta’s confusion.

Only one woman present during the encounter knows what to do. Guillermina Pacheco asks Fortunata to come to see her the next day to discuss the situation. The frank conversation between the two women is overheard by Jacinta, who is in the next room. The cruelest blow to Jacinta is Fortunata’s insistence that Juanito needs her, since she gave him the son his wife could never bear him. When Fortunata discovers the eavesdropper, her angry words show that she is still essentially of the lower class.

Later, Fortunata has a scene with Maximiliano, who is gradually losing his mind. At last he drives her out of the house. Before long, she and Juanito once again become lovers. Maximiliano tries to earn a living by working in a drugstore, but his mental state causes him to make dangerous mistakes in mixing drugs. His employer has two daughters. One is Aurora, the thirty-three-year-old widow of a Frenchman killed while fighting the Prussians in 1870. She wears clothes with a Parisian flair and soon catches the eye of Juanito, as Fortunata learns to her dismay.

In the meantime, Moreno Isla falls violently in love with Jacinta. He and Guillermina Pacheco, bribed by Moreno, try to convince her that her husband will never be faithful, but Jacinta gives Moreno no encouragement. At the same time Aurora, for her own purposes, tries to convince Juanito that his wife is in love with another man.

Fortunata is pregnant and is therefore afraid to live with Maximiliano any longer. He talks constantly of a philosophy of death; afraid, she hides herself at Aunt Lupe’s house. While looking for her, Maximiliano discovers proof that Juanito and Aurora are having an affair. He finally discovers his wife’s hiding place after Estupiñá takes the news of Fortunata’s baby son to the Santa Cruz household. No longer wanting to kill her, Maximiliano forces his way into Fortunata’s room, where he tells her what he knows about Juanito and Aurora. Although the doctor orders her not to leave her bed, Fortunata rushes out to revenge herself on Jacinta’s enemy and her own. The exertion causes her death. Before she dies, she sends a letter by Estupiñá to Jacinta. In the letter, she asks Jacinta to care for Juanito’s son.

Being compelled to acknowledge his paternity is a blow to Juanito, for it loses him his wife’s remaining esteem. He realizes sadly that his philandering brings him to old age in spirit while he is still young in years, with nothing but an empty and unhappy future before him.