The setting is Paris during Carnival, towards the end of the 19th century. ;Act 1 :Paul Aubier and his wife Angèle are guests of Georges and Marguérite Duménil. Marguérite is sceptical of the fidelity of men in marriage, and she persuades Angèle to put their husbands to the test. On their instructions, the chambermaid Hortense writes two identical letters which invite Paul and Georges to an opera ball at the
Paris Opera, where they will meet a lady with a pink domino as part of her dress. Secretly, Hortense writes a third such letter for the cadet Henri, because she wants to appear also with the pink domino. ;Act 2 :The setting is the opera ball, where everyone is masked. The old Beaubuisson, Henri's uncle, is accompanied by the singer Féodora. Of the three ladies with the pink domino, Henri sees the pink domino on Hortense, Georges sees it on Angèle, and Paul on Marguérite. Georges and Paul hope to meet the mysterious lady each in a . However, Angèle and Marguérite have arranged for a bell signal, at whose sound the lovers are to meet outside the rooms. In the confusion, Paul and Georges meet each other, and not the expected lady. Furthermore, they each then see Hortense with the pink domino, not knowing who she is. Each tries to engage the lady, but in the process, Hortense's pink domino gets scorched and ripped. Marguérite and Angèle are both unaware of the presence of Hortense. ;Act 3 :Back at his residence, Georges discovers the writing paper on which the invitation to the opera ball was written and tries to figure out the situation. Things build to the point where Georges challenges Paul to a duel. Finally, Marguérite and Angèle show their intact pink dominoes, which nominally proves the fidelity of their husbands. The role of Hortense in the plot is revealed to all. ==Film versions==