In an elegant salon of the Pontevedrian embassy in Paris, Baron Zeta, and his youthful flirtatious wife, Valencienne, are holding a birthday celebration for their royal ruler.  However, the rather naive Valencienne foolishly mislays her fan, which has a declaration of love written on it by her ardent admirer, the Count de Rosillon.  "The Merry Widow", Anna Glavari, arrives and meets the handsome Pontevedrian attaché, Count Danilovitsch, for the first time since the break up of their affair some years previously by his uncle.  Danilo's orders from Zeta are to ensure that Anna does not marry a foreigner, so that her wealth will remain with impoverished Pontevedro.

Danilo's jealousy causes him to behave badly to her in public, but when they finally dance together, Anna guesses how much he really still loves her.

At a garden party held by Anna the following evening, she announces that the garden will later be transformed to look like Maxim's.  At this point, things seem quite promising for Anna and Danilo, but not for long!  The Baron insists on spying on a couple in the summerhouse, knowing that the man to be Camille, but anxious to find out who the lady could be.  He catches a glimpse of the lady and thinks she is his wife, Valencienne.  However Anna appears and rescues Valencienne from this innocent but compromising situation by switching places with her before the door is opened for all to see.  The apparent guilt of Anna leaves Danilo desolated, and in despair he announces that he is leaving to visit Maxim's.

The garden has now been redesigned into Maxim's and some grisettes have been brought into dance, with one addition "by special request" - that Baron's wife, Valencienne.  The Baron's aide, Njegus, advises that Danilo will be brought back to see the show.  At the Baron's instigation, Danilo speaks to Anna and discovers the deception in the summerhouse, and then also establishes that the Widow has no intention of marrying any foreigner, so her wealth is safe for Pontevedro.  Anna, in the meantime, manoeuvres a declaration of love from Danilo, and the final song confirms the complete superiority of women over men, as we already acknowledge.