Ambassador Baron Zeta, is giving a ball. His home country, Pontevedro, is nearly bankrupt and he has a plan to get Danilo Danilovitch, a womanizing aristocrat and the embassy First Secretary, to marry the wealthy widow, Hanna Glawari, so that her millions will stay in Pontevedro. Zeta sees his young wife, Valencienne, flirting with the Frenchman, Camille de Rosillon, assuming she’s trying to win French support for Pontevedro. In fact, Camille has declared his love for Valencienne and writes “I love you” on her fan. Hanna arrives and the men shower her with compliments. Finally, Danilo arrives. He and Hanna reveal that they were once in love but he tells her he’s not interested in marriage now. Meanwhile, Zeta’s chief of staff, Kromov, finds Valencienne’s fan and thinks it belongs to his wife, Olga. Zeta, wanting to spare Olga the scandal, convinces him it is Valencienne’s. He then meets with Danilo and orders him to marry Hanna for the good of Pontevedro. Danilo tells him that he will not marry her but that he will keep all the Parisian potential suitors away from her. When the ladies’ choice dance is announced, Hanna selects Danilo and, after some flirtatious banter, the two finally dance. 

The next day Hanna hosts a party at her villa. Danilo arrives late and Zeta commands him to return to his mission of keeping the Parisian men from Hanna - particularly Camille. Danilo’s assistant, Njegus, reveals that Camille is already in love with a married woman. Zeta wants to know who she is, in order to marry her off to Camille, leaving Hanna free for a Pontevedrian suitor. Believing the fan to be the key to her identity, he asks Danilo to find its owner. Camille and Valencienne finally find the missing fan, and this time Valencienne writes “I am a respectable wife” on it. Observed by Njegus, they disappear into the summerhouse. When Zeta arrives to meet Danilo, Njegus prevents him from entering the pavilion to protect Valencienne’s secret and instead sneaks Hanna in to take her place. Hanna emerges with Camille, announcing their engagement. A furious Danilo departs for Maxim’s, which Hanna takes as proof of his love.

Arriving at Maxim’s in search of Danilo, Camille and Valencienne sneak off to one of the private rooms. Zeta and the other Pontevedrians appear, and the Grisettes - among them a dressed-up Valencienne - entertain the crowd. Eventually both Danilo and Hanna arrive. He forbids her to marry Camille. When she explains that she was merely safeguarding another woman’s reputation, he is delighted but still won’t declare his love. As the guests reassemble, Danilo announces that Hanna will not marry Camille, but he will not reveal the identity of Camille’s secret lover. Njegus produces the missing fan, which he found in the pavilion. Zeta finally recognizes it as his wife’s, declares himself divorced, and proposes to Hanna - who informs him that, according to her late husband’s will, she will lose her fortune if she remarries. At this, the other men lose interest in Hanna, except Danilo, who finally declares his love and asks Hanna to marry him. She accepts and amends her account of the will: upon remarrying, her fortune will pass to her new husband. Valencienne asks Zeta to read the other side of her fan - which reads, “I am a respectable wife.” With the couples united, the men are left to wonder about the mystery of women.