In the Mayor's garden, the disguised Marchioness seeks her former lover Belfiore, who had stabbed and deserted her, Ramiro has been rejected by Arminda and the Mayor, Don Anchise, loves the supposed Sandrina. Roberto, alias Nardo, wants to marry Serpetta, who wants to marry the Podesta. Belfiore is now in love with Arminda, who intends to keep him. The second act, in the house, finds Nardo (Roberto) wooing the reluctant Serpetta, Sandrina (Violante) describing her death to Belfiore, the Mayor attempting to attract Sandrina and Ramiro trying to have Belfiore arrested for murder of Violante (Sandrina). with the Mayor refusing to allow any niece of his to marry a murderer. In the following scene she tries to exonerate him by claiming that there was no murder. Finally abandoned through Arminda's jealous agency in a wood, she hears the others, each mistaking the other's identity. Now Sandrina, under stress, imagines herself a shepherdess, Cloris, wooing her Thyrsis, Belfiore, whom the Mayor and Ramiro both challenge to a duel, a procedure to which Nardo, Serpetta and Arminda object. Sandrina now imagines herself Medusa, while Belfiore thinks himself Hercules. Still deluded in the third act, Belfiore now addresses Nardo (Roberto) as Venus, while he himself is Mercury. Sandrina, now thinking herself Herminea, declares that she loves and will marry Nardo. In the last scene all, as might be expected, comes right. Sandrina and Belfiore are restored to their senses and united, Serpetta agrees to Nardo's suit and Arminda is happy with Ramiro.

In a plot of obvious complexity, as characters assume other identities, either consciously or in madness, Mozart, in this his second comic opera, creates more rounded figures out of the stuff of two-dimensional comedy. The overture may be heard in the concert-hall, while the musical substance of the opera allows Arminda moments of high drama, with more high flown writing for the nobility and an appropriately earthier style of relative simplicity for the lower classes.