At the home of the Podestà (Mayor) Don Anchise, preparations are underway for a grand dinner to celebrate the engagement of his neice Arminda and Contino Belfiore. But not all feel like partaking in the merriment. With the appearance of the new gardener Sandrina in the household, the amorous relations between its members become terribly complicated. No-one guesses that Sandrina is actually the Marchioness Violante Onesti in disguise. One year earlier, in a pique of jealousy Contino Belfiore wounded Violante. At the time, the Contino imagined that it was a fatal wound and fled. Don Anchise is distracted by the pretty-looking gardener, which rouses the jealous indignation of Serpetta: the maid had set her sights on the rich bachelor. The Gardener-Marchioness is accompanied by the servant Roberto who calls himself Nardo and claims to be a gardener and Sandrina's cousin. Nardo is in love with Serpetta, but she does not reciprocate his feelings. For the sake of a noble title, Arminda has rejected the suitor Ramiro. Ramiro is despondent.
Upon his arrival the Contino lavishes compliments on his fiancée. The news of the impending marriage of the Contino to Arminda so perturbs Sandrina that she faints. Assisting Sandrina, Belfiore recognises that the gardener is actually the Marchioness. His former love erupts with new fervour.
The lovers shower each other with reproaches: Ramiro and Arminda, Arminda and the Contino. Nardo reproaches Serpetta with her insensitivity and Sandrina-Violante accuses the Contino of betrayal. The Podestà finds the gardener with the Contino and is indignant. The resolve with which Sandrina spurns Don Anchise's pretensions moves him. Arminda asks that the wedding be hurried on, but the affair suddenly becomes complicated: Ramiro hands Don Anchise a letter, which claims that the Contino Belfiore is wanted on suspicion of murdering the Marchioness Onesti. The wedding is postponed. The Podestà interrogates the Contino. Wishing to save her beloved, the pretend gardener reveals her true name, but immediately takes it all back, telling the Contino that she merely made use of her similarity to the Marchioness. Violante is still insulted by the Contino's betrayal. Overflowing with emotion, Violante flees from the house, and all follow her. In the darkness of the night it is difficult to recognise one another, and it all becomes a tangled mess until Ramiro illuminates the situation with torches. The Contino and the Marchioness behave in the most peculiar fashion, as if they were mad.
The Contino and the Marchioness are still behaving oddly. The Podestà decides to let the Contino go and become betrothed to Sandrina, which leads to a new bout of jealousy from Serpetta. Arminda insists on marrying the Contino, while Ramiro is determined in his intention to win Arminda's hand. Belfiore and Violante finally make peace and are cured of their "madness". The gardener again reveals herself to be the Marchioness. The three couples in love come together happily: the Marchioness Onesti with the Contino Belfiore, Arminda with Ramiro and Serpetta with Nardo-Roberto. Only Don Anchise is left without a partner: he will wait for a bride as faithful in love as Sandrina has been.