Figaro and Susanna, the servants of Count and Countess Almaviva, are preparing for their imminent wedding. As Figaro measures their assigned quarters for a bed, Susanna reveals that she is being pursued by the Count despite her engagement to Figaro. Dr. Bartolo and Marcellina enter, conspiring to hold Figaro to a contract he made to marry Marcellina if he failed to pay back a loan. Next comes the pageboy Cherubino; he tells Susanna of his desire for all women, especially 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. The indiscreet Basilio talks of the Count’s love for Susanna and Cherubino’s love for the Countess. Enraged, the Count emerges from hiding, discovers Cherubino, and attempts to end the boy’s flirtatious ways with an army commission to be effective immediately.

The Countess plots with Susanna to expose her husband’s philandering. Susanna will write the Count agreeing to a rendezvous, but they will send Cherubino in her place, dressed in women’s clothes. As they work together to disguise Cherubino in drag, the Count arrives and Cherubino hides in a nearby closet. The Count, hearing noises, suspects their ruse and leaves to fetch tools with which to break open the door. To escape, Cherubino leaps out the window while Susanna takes his place. When the Count returns and finds Susanna in the closet, he is forced to apologize. Figaro arrives, then the irate gardener Antonio, whose flowerbed was destroyed by Cherubino’s leap. Figaro quickly claims that it was he who leapt from the window, but the Count is suspicious when Antonio produces Cherubino’s dropped army commission. Marcellina, Bartolo and Basilio burst in, demanding that Figaro honor the contract for marriage.

Figaro stalls Marcellina by telling her that he is of noble birth and cannot marry without his parents’ consent. As proof he reveals a birthmark on his arm — whereupon Marcellina realizes that she and Bartolo have found their long-lost son. With the Count still intending to bed Susanna on her wedding night, she and the Countess come up with a plan to thwart him. To bait their trap, the Countess dictates a note for Susanna to pass the Count after the double-wedding of Susanna to Figaro and Marcellina to 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, and a series of mistaken identities ensues. The Count attempts to seduce “Susanna,” who is actually the Countess in disguise. Figaro eventually realizes his wife is faithful and the Count is publicly embarrassed when he accuses Figaro of romancing his wife, only to find out it is Susanna. Chastened, he begs for the Countess’s forgiveness, which she bestows.