In Paris, Mirko Zeta, ambassador of the Principality of Pontevedro, throws a party to welcome the wealthy widow Hanna Glawari who has just arrived in the capital. While his wife Valencienne tries to stave off the advances of the young Camille Roussillon, the ambassador sets about ensuring that Hanna Glawari gets remarried as quickly as possible to one of their fellow countrymen to prevent her fortune from leaving Pontevedro, which would result in that country’s bankruptcy. Count Danilo seems to be the ideal candidate, but he bluntly rejects the idea. In reality, he knows Hanna. Indeed, he would have married her long ago had an aristocratic uncle not been opposed to the union on the grounds that he would be marrying beneath his station. Now, Danilo is eager to prevent people from thinking that he would only be marrying the young woman because of her fortune.
Hannah in turn throws a party. She sings the Song of Vilya for her guests, a ballad about a young dryad who seduces a hunter. In the face of Danilo’s repeated rejections, the other gentlemen declare themselves candidates for marriage. Meanwhile, Valencienne, who is on the verge of yielding to Camille’s incessant wooing, agrees to meet the latter in the small lodge. Her husband sees them and follows. However, when he opens the door to the lodge he discovers Hanna who has had the time to take Valencienne’s place next to Camille. The ruse deceives the ambassador who shudders at the prospect of having the young widow marry a Frenchman. Danilo is also furious. His jealousy finally shows through, thus revealing his true feelings to Hanna.
For the reception at her home, thanks to the "grisettes" and their songs Hanna recreates the atmosphere of Maxim’s. Danilo, for his part, returns from the real Maxim’s, having found no one there. At the same time, Mirko Zeta receives a calamitous dispatch informing him that only the young widow's fortune can save Pontevedro from bankruptcy. He begs Danilo who easily convinces Hanna not to marry Camille. She in turn explains the deception at the lodge: they can now declare their love in the famous duet: “Lippen schweigen”. Zeta finds his wife’s fan in the lodge. He demands a divorce and asks Hanna for her hand. However, he quickly withdraws the offer once he learns that the widow will lose her fortune in the event that she re-marries. Danilo can now unambiguously ask Hanna to marry him. But he can no longer have a change of heart, even when he learns that the widow will have to relinquish her fortune… to her new husband!