Figaro, former barber of Seville, measures the room he will occupy after his marriage to Susanna. Both are in the service of Count Almaviva, and when Susanna warns her fiancé that the Count has given them this room near his own because he has designs on her, Figaro vows to outwit his master (“Se vuol ballare”). After they leave, Dr. Bartolo, the Countess’s onetime guardian and suitor, arrives with his housekeeper, Marcellina. Bartolo is eager for revenge on Figaro, whose machinations caused him to lose his ward to Almaviva. Knowing that Figaro once gave Marcellina his promise of marriage as collateral on a loan, Bartolo persuades her to foreclose (“La vendetta”) and leaves. When Susanna returns, she trades insults with her would-be rival (“Via resti servita”), who storms out. The skirt-chasing page Cherubino steals in, begging Susanna’s protection from the Count, who has caught him flirting with Barbarina, the gardener’s daughter. After pouring out his amorous enthusiasm (“Non so più”), he hides as the Count enters to woo Susanna. Interrupted by the arrival of the music master, Don Basilio, the Count in turn hides, but he steps forward when Basilio hints that Cherubino has a crush on the Countess. Just as the Count discovers the hapless Cherubino, Figaro brings in a group of peasants to salute their lord for abolishing the droit du seigneur, an old custom giving the local landowner the first night with any bride among his retainers. Feigning good will, the Count drafts Cherubino into his regiment. Figaro teases the boy about his new military life (“Non più andrai”).
In her boudoir, the Countess laments the waning of her husband’s love (“Porgi, amor”). When Figaro and Susanna arrive with news of the Count’s machinations, the three plot to chasten him. Cherubino, disguised as Susanna, will keep an assignation with the Count. When Figaro leaves, the page comes to serenade the Countess with a song of his own composition (“Voi che sapete”). While dressing the boy in girl’s clothes, Susanna goes out for a ribbon, and the Count knocks, furious to find the door barred. The Countess locks Cherubino in a closet before admitting her husband. The jealous Count hears a noise; the Countess insists it’s Susanna, but he doesn’t believe her. Taking his wife with him, he goes to fetch tools to force the lock. Susanna, who has slipped in unnoticed during their confrontation, helps Cherubino out a window and takes his place in the closet, baffling both Count and Countess when they return. As the Count tries to make amends, the gardener, Antonio, appears, complaining that someone has stepped in his flower bed. Figaro, arriving to say the wedding ceremony is ready to begin, claims it was he who jumped from the window and fakes a twisted ankle. When the Count asks him about a paper found among the geraniums, Figaro, prompted by the women, correctly identifies it as Cherubino’s commission. Bartolo and Basilio burst in with Marcellina to press her claims against Figaro. The Count gladly postpones the wedding, pledging to judge the case himself.
At the Countess’s prompting, Susanna promises the Count a rendezvous (“Crudel! perchè finora”), but his suspicions are aroused when he overhears her assuring Figaro that the case is won. Enraged, he vows revenge (“Vedrò, mentr’io sospiro”). Alone, the Countess hopes to revive her husband’s love (“Dove sono”). Marcellina now demands that Figaro pay his debt or marry her, but a birthmark proves he is her long-lost son by Bartolo, and the parents call off their suit, confounding the Count (“Riconosci in questo amplesso”). The conspiracy continues: the Countess dictates a note from Susanna, inviting the Count to the garden (“Che soave zeffiretto”). Peasants, among them Cherubino, disguised as a girl, bring flowers to their lady. Figaro arrives, and, as the wedding ceremony at last takes place, Susanna slips the note, sealed with a pin, to the Count.
The pin is meant to accompany the Count’s reply, but Barbarina, his messenger, has lost it in the dusky garden (“L’ho perduta, me meschina”). She explains her predicament to Figaro, who, unaware of the ladies’ latest plot, thinks Susanna has betrayed him. He gives Barbarina another pin, planning to ambush his bride with the Count, then turns to his mother, Marcellina, for comfort. The crafty Basilio says it pays to play the fool. Figaro, left alone, curses women for their duplicity (“Aprite un po'”), then hides when Susanna appears, rhapsodizing on her love for Figaro without naming him (“Deh vieni”). Figaro is beside himself, assuming her serenade is meant for the Count. Susanna and the Countess secretly exchange dresses, and in the darkness both Cherubino and the Count woo the Countess, thinking her to be Susanna (“Pian, pianin le andrò più presso”). Figaro at last perceives the joke and gets even by wooing Susanna in her Countess disguise, provoking and then pacifying her. When the Count returns, he sees Figaro flirting with what appears to be the Countess. He calls the whole company to witness his judgment but is silenced when the real Countess appears and reveals the ruse. She grants the Count’s plea for forgiveness (“Contessa, perdono”), and everyone celebrates.