Count Almaviva has fallen head over heels in love with Rosina, the ward of Dr. Bartolo of Seville. To show his love, he secretly sings beneath her window every morning. By chance, Almaviva meets Figaro, who, constantly in debt, ekes out a living in Seville as a barber and as Bartolo’s assistant and general factotum. Rosina’s lot is an unenviable one: Bartolo, her tight-fisted and irascible guardian, wants to marry her himself for her dowry. She, however, has already fallen in love with the young count, who introduced himself to her as a student named Lindoro. For an appropriate reward, Figaro offers the count his services in his quest to win Rosina.
Rosina spends her time oscillating between yearning, defiance, and boredom. The music teacher Basilio turns up to warn his friend Bartolo that Almaviva is in the city. He argues that there is only one way to get rid of the count: slander. Rosina hears from Figaro that Lindoro wants to marry her, and writes a quick note to her beloved.
Bartolo’s suspicions are aroused when he sees ink on Rosina’s hands and discovers that a piece of paper is missing. Following Figaro’s advice, Almaviva forces his way into Bartolo’s house, pretending to be a drunken soldier. Bartolo, however, is able to prove that he is exempt from billeting troops. The quarrelling leads to general turmoil and finally alerts the local guards. As Almaviva is about to be arrested, he reveals his identity to the officer. Bartolo is dumbfounded by this unexpected outcome.
Claiming to be the music teacher Alonso – substituting for the allegedly ill Basilio – Almaviva comes to visit Bartolo. He succeeds in dispelling Bartolo’s suspicions. Rosina of course, recognizes Alonso as her Lindoro, and a music lesson is improvised to keep up the pretence. Bartolo, naturally, finds modern music abominable. Figaro manages to steal the key to the balcony door from Bartolo. To everyone’s surprise, Basilio enters the room, baffled by the group’s concern for his health. Only the full purse of the count can persuade him to leave. Rosina is enthusiastic about the plan to elope at midnight. Believing that something is being concealed from him, Bartolo sends everyone away.
Bartolo’s Servant Berta is angry: her hopes of becoming Bartolo’s wife are being dashed by his courting of Rosina. Bartolo decides to act, and sends Basilio to the notary to have the marriage contract drawn up. He tells his ward that he will marry her to Almaviva and no one else. In her despair, Rosina declares herself willing to marry Bartolo right away. She also tells him that she is to elope at midnight, and Bartolo rushes off to alert the guards. In the meantime, the count and Figaro have climbed over the balcony and entered the house. Rosina is delighted to learn that Almaviva and Lindoro are one and the same. Just as they are about to leave, Basilio arrives with the notary. A ring and a gun persuade Basilio to sign the marriage contract between Almaviva and Rosina. Bartolo, too, decides to make the best of things, when the count announces to turn Rosina’s dowry into a gift for him.