Outside the house of Dr. Bartolo in Seville. Accompanied by the servant Fiorello and a group of hired musicians, Count Almaviva serenades (“Ecco ridente”) his beloved Rosina from beneath her window. Along comes Figaro—barber, doctor, matchmaker, and self-styled “factotum” to all of Seville (Aria: “Largo al factotum”). Figaro informs the Count, his former employer, that Rosina is the ward of old Dr. Bartolo. Rosina drops a letter from her window asking her mysterious serenader to identify himself. In a second serenade, the Count fabricates an identity as a poor student named “Lindoro” (since he wants to be loved for himself, and not his riches). After observing Dr. Bartolo make plans to marry Rosina himself, the Count and Figaro, in the duet “All’idea di quel metallo,” plot to win Rosina for the Count instead. In order to get the Count into Bartolo’s house, Figaro will disguise the Count as a drunken soldier to be billeted there.
Inside the house, Rosina declares her intention to have the man she wants (Aria: “Una voce poco fa”). Dr. Bartolo is furious with Figaro, because the medication Figaro prescribed for Bartolo’s servants has reduced the household to chaos. Don Basilio, Rosina’s singing teacher, warns Bartolo that Count Almaviva has designs on Rosina, and in the aria “La calunnia,” Basilio suggests that slander would be the best way to get rid of him. Bartolo insists they draw up his marriage contract to Rosina immediately. Figaro, who overhears their plot, warns Rosina, promises to deliver a note from her to “Lindoro,” and departs. The suspicious Dr. Bartolo deduces that Rosina has written someone a letter and heckles her in the aria “A un dottor della mia sorte.” The Count bursts into the house in his drunken soldier disguise and insists that he is to lodge there, despite Dr. Bartolo’s strenuous objections. In the frolicking Act I finale, their ensuing quarrel becomes a public disturbance.
Later that afternoon. Having gotten rid of the drunken soldier, Dr. Bartolo opens his doors to the Count—disguised this time as Don Alonso, Don Basilio’s substitute music teacher (Duet: “Pace e gioia sia con voi”). “Don Alonso” allays Dr. Bartolo’s suspicions by giving Bartolo the letter Rosina wrote to “Lindoro,” and describes his plan to slander Lindoro, who is clearly pursuing women on the Count’s behalf. At her singing lesson, Rosina sings an aria from The Useless Precaution, her favorite opera. Meanwhile Figaro, who is giving Dr. Bartolo a shave, manages to steal a key to the house. Don Basilio enters suddenly, beginning a quintet; but the others quickly boot him out the door, and quintet becomes quartet. Dr. Bartolo finally discovers the lovers’ plot and kicks the Count and Figaro out of his house. Berta, Dr. Bartolo’s maid, hates her life (Aria: “Il vecchiotto cerca moglie”). Dr. Bartolo asks Don Basilio to fetch the notary so Bartolo can marry Rosina at once. Bartolo shows Rosina her letter and tells her that her “Lindoro” is really wooing her on behalf of another man, Count Almaviva. Shocked and unhappy, Rosina agrees to marry Dr. Bartolo.
After a brief storm, Figaro and the Count, using the stolen key, enter with the intention of liberating Rosina. When Rosina refuses to go with them, “Lindoro” reveals his true identity (Trio: “Ah! Qual colpo”). Basilio is threatened and bribed into witnessing the marriage of Almaviva and Rosina. Bartolo concedes defeat and blesses the lovers.