The action of La Duquesa del Bal Tabarin takes place in Paris. Frou Frou is a dances in the famous night club Bal Tabarin, with whom the Duke of Pontarcy falls in love. They get married, but she soon becomes bored and misses her previous gay and frivlolous life. Surprised by the duke with one of her admirers he starts divorce proceedings. The court grants Frou Frou a term of three months, at the end of which, if she has observed the proper decorum and her conduct is above reproach, she will get half a million francs from the Duke, and the divorce will become final. The action starts when the three months are almost at an end, and the Duchess is making a date with Octavio, her current admirer, for 12:00 that night, when the term expires and she is finally free. The Duke, who is also minister of communications, has in the meanwhile, organized a new telephone switchboard and tries to date Ketty, one of the beautiful telephone operators, who in turn is betrothed to Octavio, Prince of Chantal, whom she believes to be a commoner.
The Duke comes to visit the new office very often, with the excuse of inspecting its service and in one of these visits he asks Ketty to go with him to the Bal Tabarin. Ketty refuses at first, but when she discovers Frou Frou's affair with Octavio, she decides to accept the Duke's invitation. At the club, both couples meet, and the Duke is furious at seeing his wife back in the old haunts. The Duchess tells him indignantly, that her term of probation has expired, as it is the 28th of February, last day of the month. But, as it so happens, it is leap year and the last day is really the 29th. Frou Frou, deperate, threatens to kill herself, and the Duke forgives her, to prevent a scandal. Octavio and Ketty also decide that they truly love each other, and everything ends well.