Figaro and Susanna, Count Almaviva's servants, are preparing for their imminent wedding, when Susanna reveals that she is being pursued by the count.
Meanwhile Bartolo and Marcellina are trying to hold Figaro to a promise he made to marry Marcellina if he failed to pay back a loan to Bartolo.
The page Cherubino tells Susanna of his love for the countess, just as the count arrives.
Cherubino hides and overhears Almaviva's attempted seduction of Susanna. Don Basilio's arrival forces the count into hiding as well, which, in turn, forces Cherubino to move from his hiding place.
The indiscreet Basilio talks of the count's love for Susanna and of Cherubino's love for the countess. The count then emerges from hiding, precipitating a scene that ends with Cherubino being packed off by the count to join the army.
The countess, distressed by her husband's philandering, plots with Susanna to humiliate him: Susanna will write to the count, agreeing to a rendezvous, but they will send Cherubino in her place, dressed in women's clothes. As Cherubino is being disguised, the count arrives, and Cherubino is pushed into an adjoining closet.
The count, hearing noises, leaves to fetch tools with which to break open the door. In the meantime Cherubino leaps out of the window while Susanna takes his place.
When the count returns and finds that it is Susanna
in the closet, he is forced to apologize for his suspicions.To add to the confusion Figaro then arrives, then an irate gardener (Cherubino's leap has destroyed some plants), and finally Bartolo with Marcellina, who wants her claim on Figaro honoured.
Figaro stalls Marcellina by telling her that he is of noble birth and cannot marry without his parents' consent. He reveals a birthmark on his arm, whereupon Marcellina realizes that she and Bartolo have found their long-lost son. They embrace just as Susanne enters. She is furious, but once the situation is explained she joins in the celebration.
After the countess has dictated the note that Susanna is to pass to the Count, a double wedding is prepared - Susanna and Figaro being joined by Marcellina and Bartolo.
Figaro encounters Barbarina, the gardener's daughter, who inadvertently reveals that Susanna has received a message from the count. Figaro assumes the worst, and decides to catch his errant wife when she arrives for her tryst in the garden.
The countess and Susanna appear in each other's clothes (Susanne has now taken Cherubino's place in the subterfuge) and much mistaken identity ensues in the darkness (Cherubino is there to meet Barbarina too).
Eventually Figaro realizes his wife is faithful and the count realizes he has been tricked. He begs for his wife's forgiveness, and receives it.