A castle–the home of the Count and Countess Almaviva–near Seville, in the late 1700s
Figaro and Susanna, the valet and maid of Count and Countess Almaviva, are to marry today. Susanna tells Figaro that the Count has been trying to seduce her, and Figaro vows to teach the Count a lesson. Meanwhile, Dr. Bartolo, still seeking vengeance on Figaro for the events of The Barber of Seville, consults with his former servant, Marcellina. She is determined to collect on an old loan made to Figaro. According to the terms, Figaro must either pay her back or marry her. Marcellina fights with her younger rival, Susanna.
The teenage page Cherubino wants Susanna to plead on his behalf with the Countess to reinstate him in the Count's good graces—the Count has banished Cherubino from the castle after finding him with the gardener Antonio's daughter, Barbarina. They hear the Count approaching, and Cherubino hides. The Count attempts to arrange an assignation with Susanna, and he, too, hides when Don Basilio, the music teacher, arrives. When Basilio gossips about Cherubino's crush on the Countess, the jealous Count steps forward. He is telling the story of how he found Cherubino with Barbarina when he discovers Cherubino in yet another compromising situation. Figaro enters and tries to force the Count to marry him to Susanna on the spot. But the Count delays the wedding and orders Cherubino to enlist in his personal regiment in the army.
The Countess is heartbroken by her husband's philandering. Susanna sympathizes with her. Figaro enters and divulges his schemes. He has sent the Count an anonymous note telling him that the Countess is expecting a lover while the Count is out hunting. Figaro hopes to keep the Count embroiled in this ruse to deflect his attention from Marcellina's troublesome claim. Figaro also asks Susanna to arrange a rendezvous with the Count later on that evening in the garden; Cherubino, dressed as a girl, will go in Susanna's place. The Count will be caught in the act and forced to mend his ways.
The Countess and Susanna begin to disguise Cherubino. Susanna steps out for a moment. The Count arrives in a jealous fury, having read the anonymous note. He knocks on the bedroom door and finds it locked. The Countess, terrified, hides Cherubino in the closet and then lets the Count in. Susanna re-enters, unnoticed. The Countess refuses to unlock the closet, so the Count leaves, taking the Countess with him, in search of tools to break the lock. Susanna helps the boy escape through the window, and then she hides in the closet, surprising both the Count and Countess when they find her there. Figaro arrives and tries to get everyone to come to the wedding festivities. When the gardener enters and claims someone has jumped out of the window, Figaro takes the blame. Marcellina bursts in with Bartolo and Basilio and demands her case against Figaro be heard.
The Countess alters Figaro's plan: Susanna will ask the Count to meet her in the garden that evening, but instead of Cherubino the Countess will go in her place. The Count eagerly agrees to meet Susanna, but he hears her tell Figaro that they have already "won the case" and he is once again filled with suspicion.
Don Curzio, chosen by the Count to hear Marcellina's case, judges that Figaro must either pay off the debt or marry Marcellina. Figaro claims that, as the son of an aristocrat, he cannot marry without the consent of his parents, and since he was a foundling, he doesn't expect to be able to find them. Hearing the story of his childhood abduction, Marcellina realizes that she is Figaro's mother and that his father is Dr. Bartolo. Susanna re-enters with money the Countess has given her to pay off Figaro's debt. Enraged at first at seeing Figaro embrace Marcellina, she calms down when she understands the true situation.
The Countess remembers her love for the Count when they first met, and still cares enough to brave danger to win him back. She dictates a note for Susanna to give to the Count, specifying the location of their supposed rendezvous later that evening in the garden. During the double wedding (of Figaro to Susanna and Bartolo to Marcellina), Susanna slips this note to the Count. The Count is to return a pin used to seal the note as an acknowledgment that he will meet her. He gives the pin to Barbarina to give to Susanna.
Barbarina is looking both for Cherubino and for the pin the Count gave her. She tells Figaro about Susanna's pin, and he jumps to the conclusion that Susanna really is planning to betray him. Crushed, he hides in the garden and plans his revenge. Susanna and the Countess arrive and switch cloaks to disguise themselves as each other. Their scheme to fool the Count is disrupted by the inopportune arrival of Cherubino. Figaro eventually realizes what is going on and gets even with Susanna by wooing her in her Countess disguise. Mistaking Susanna for his wife, the Count attempts to "expose" her, but when the real Countess appears, the Count is the one who must ask for forgiveness.