The action takes place in Naples in the rich house of fifty-two-year-old don Domenico Soriano, a successful entrepreneur. In the room are Don Domenico himself, Donna Filumena Marturano, the woman with whom he lived for the last twenty years, Donna Rosalia Solimene, a seventy-five year old woman who shared the most sad moments in Filumena's life, as well as Alfredo Amoroso, an elderly servant of Don Domenico. Once Don Domenico took Filumena to him from the lowest strata of Neapolitan society; at that time she worked in a house of tolerance. After the death of his wife, after two years of their acquaintance, Filumena hoped that don Domenico would marry her, but this did not happen. So she lived in his house with Rosalia Solimene as a half-lover, half-slave, and in addition, she checked the work of his factories and shops, while the owner himself had fun in London and in Paris, at horse races and with women. Finally, Filumena decided to put an end to her disenfranchised position: she pretended to be terribly ill, that she had a death throes, called the priest ostensibly for the last communion, and then asked don Domenico to fulfill the dying woman’s desire and allow her on her deathbed to be matched with him marriage. As soon as don Domenico fulfilled her request, Filumena immediately jumped out of bed in good health and announced to him that now they are husband and wife. Don Domenico realized that he fell for her bait and is completely in her power. Now he is furious and promises that he will spare no money or strength to destroy and crush the insidious.
During an angry bickering, Filumena accuses Domenico of the fact that he always treated her low, and even when he thought that when she died, at her own bed, she kissed some girl who was brought into the house under the guise of a nurse. At the end of her accusatory speech, Filumena states that she has three sons, whom Domenico does not know about, and in order to raise them, she often stole money from him, and now she will achieve that they will bear the name of Soriano. Domenico and Alfredo are stunned. Rosalie has long been aware of this. Filumena asks Domenico not to be very scared, because the children are not him and are already adults. She often sees them, but her sons do not know that she is their mother. One of them, with her help, became a plumber, he has his own workshop, he is married and has four children. The second, his name is Riccardo, is holding a men's underwear shop; the third, Umberto, became an accountant and even writes stories for the newspaper.
Alfredo confusedly reports that the waiters from the restaurant came and brought the dinner that Domenico ordered in the morning, thought that by evening he would already become a widower and could have fun with young Diana, just the one with whom she was kissing at the bedside of the “dying” Filumena. Soon, Diana herself appears. She is very elegant and looks down on everyone. At first she does not notice Filumena, chatting about her plans, but when she sees her, she gets up and backs away, Filumena treats her rather abruptly and drives her out. Domenico swears that while he is alive, the legs of the sons of Filumena will not be in his house, but she is sure that he did it in vain, for she knows that she will not be able to keep her word; someday, if he does not want to die damned, he will have to ask her for alms. Domenico does not believe her and still threatens to deal with her.
The next day, Alfredo, who sat all night with Don Domenico at the parapet of the Caracciolo monument, coughs and asks the genre Lucia to bring him coffee. While he waits, Rosalia leaves the Filumena's room. She must, on behalf of her mistress, send three letters. Alfredo is trying to find out to whom they are addressed, but Rosalia strictly keeps a confidential secret. Don Domenico, who returned from the street, himself drinks the intended Alfredo coffee to the great displeasure of his servant. Soon Filumena comes out of the bedroom and orders to prepare two rooms for her two single sons. Married remains to live where he lived before. Lucia has to move to the kitchen with all her things.
While the women are busy preparing, Diana and Nocell's lawyer enter the house. They want to talk with don Domenico, and all three of them retire to the owner’s office. Meanwhile, Umberto, one of Filumena's sons, enters the dining room and writes something. Riccardo who appeared after him does not pay any attention to him and immediately begins to flirt with Lucia. The last is Michele, the third son. Riccardo behaves quite defiantly; his manner of holding leads to the fact that Michele is forced to fight with him. Umberto tries to separate them. Behind this scuffle, Filumen also catches them. She wants to talk to them seriously, but this is hindered by the invasion of a contented Domenico, Diana and a lawyer. Nocella’s lawyer explains to Filumena that her act was illegal and that she had no rights to don Domenico. Filumena believes the words of a lawyer, but calls from the terrace of three young people, tells them about her life and admits that she is their mother. All three are very excited. Michele is glad that his children had a grandmother, whom they had been asking about for so long. Since Filumen is about to leave Don Domenico's house, he offers her to move to him. She agrees, but asks her sons to wait for her downstairs.
Left alone with Domenico, she informs him that one of these young people is his son. To say which, she refuses. He does not believe her, being convinced that if she had ever expected a child from him, she would definitely use this to marry him. Filumena answers that if he knew about the alleged child, he would have made him kill. Now, if his son is alive, then this is only her merit. In the end, she warns Domenico that if the children find out that he is the father of one of them, she will kill him. Ten months after the previous events, don Domenico, who managed to divorce Filumena, is now really going to marry her. During this time, he has changed a lot. There are no more imperious intonations or gestures. He became soft, almost submissive.
Three sons Filumena appear in the room, who came to her wedding. While their mother is not there, Domenico talks with them, trying to determine by their behavior and habits which of them is his son. However, it is difficult for him to make a choice, since all of them, like him, like girls, but not one of them knows how to sing, although Domenico himself, in his youth, gathered with friends, loved to sing, then serenades were in fashion, Filumena came out of her room ; she is in a wedding dress, very nice and looks younger. Domenico asks the young people, along with Rosalia, to go to the dining room and have a drink, and he resumes with the bride a conversation on a topic that has long tormented him: he is interested in who of the three is his son. He asks her for “alms”, which was predicted by Filumen.
All these ten months he came to her, in the house to Michele, and tried to talk to her, but they kept telling him that Filumena was not at home, until, at last, he came and asked her to marry him, because he understood that loves her and cannot live without her. Now, before the wedding, he wants to know the truth. Filumena arranges a test for Domenico: at first she admits that his son is Michele, a plumber. Domenico immediately tries to come up with something that could improve his son's life. Then she assures him that his son is Riccardo, and then admits that he is Umberto, but does not tell the truth. She proved to him that if Domenico finds out who his real son is, she will set him apart and love him more, and the rest will suffer or even kill each other. Their family has found fullness too late, and now it needs to be valued and protected. Domenico agrees with Filumena and admits that children are children, whoever they are, this is a great happiness; let everything remain the same and everyone go their own way. After the wedding ceremony, Domenico promises young people that he will love them equally, and shines with happiness when all three, saying goodbye, call him dad.
