In 1906, during the Giolitti era, Baron Ottone Spinelli degli Ulivi, nicknamed Zazà, despite his noble lineage, found himself in a precarious financial situation due to his dissolute life spent among theatre showgirls. Despite his poverty, Zazà continues to live a noble life, staying in a hotel (without paying) and assisted by his devoted servant Battista. When he is threatened by a creditor called Bernasconi, who has discovered his forgery of a maturing promissory note, Zazà is forced to make a decision.
Reluctantly, Zazà accepts Battista's advice to seek a loan from his brother Pio degli Ulivi, who runs a tailor's shop frequented mainly by religious people. The two brothers do not have a good relationship because of Zazà's dissolute lifestyle, which Pio considers immoral. When Zazà comes to his brother's house to ask for help, he is thrown out.
In an outburst of despair, Zazà makes Pio believe he wants to commit suicide. Back home, Zazà receives an unexpected visit from soubrette Patrizia, who pretends to seduce him in order to show him her acting skills and ask him to help her get a part in a magazine.
Zazà explains to Patrizia that the show has been abandoned by the producer, but during their discussion Pio arrives, worried about his brother's alleged suicide threats. In order to get money from Pio and manage to cover the bill, Zazà decides to make his brother believe that Patrizia is his daughter. Pio, having assessed the situation, decides to invite Zazà, the alleged niece and Patrizia's alleged boyfriend to his house.
While staying at his brother's house, Zazà disguises himself as a priest in order to fool Bernasconi into believing he is the twin brother of the real debtor in order to obtain an extension of payment. However, a series of misunderstandings occur, and Pio mistakenly convinces himself that he is Patrizia's father, even going so far as to disguise himself as a priest and being unmasked, resulting in his customers abandoning him.
In the end, Zazà manages to settle the bill and forces his brother to finance the play, with Patrizia as prima donna.