Act One
It is dawn in Seville. Count Almaviva is in love with Rosina, the ward of Dr. Bartolo. The Count serenades the young woman, but she fails to come to her window. Figaro, barber and jack-of-all-trades, enters to offer his help. Bartolo emerges, and Figaro and Almaviva overhear his plans to marry Rosina himself. Almaviva serenades her again, introducing himself as a poor student named Lindoro, not wanting Rosina to marry him for his rank. She briefly responds before the shutters are abruptly closed. Figaro concocts a plan to get the Count into her house disguised as a drunken soldier.

Inside the house, Rosina has written a love letter to Lindoro. She asks Figaro for help, but their conversation is interrupted by the entrance first of Dr. Bartolo and then by her music teacher, Don Basilio. Don Basilio warns Bartolo that a certain Count Almaviva has arrived in town and is interested in Rosina. He advises the doctor to spread slanderous rumors about the Count to drive him out of town. Bartolo replies that he would rather marry Rosina that very day and the two men go off to draft the contract.

Figaro tells Rosina of "Lindoro," and she gives him the letter she has already written to her unseen lover. Bartolo returns, suspecting something is wrong. The Count enters, pretending to be a drunken soldier ordered to lodge in Bartolo's house. In the ensuing uproar, Almaviva slips Rosina a note. The commotion attracts the attention of the city guard who attempts to arrest Almaviva. The Count takes him aside and informs him of his rank. Almaviva is immediately released, dumbfounding everyone.

Act Two
The Count again enters Bartolo's house, now disguised as Rosina's substitute music teacher, Don Alonso. Bartolo is suspicious, but Almaviva tricks him into thinking he is his ally. Rosina enters and the two proceed with their "music lesson" while Bartolo dozes. Figaro arrives to shave Bartolo, in the process obtaining the key to the balcony, so the lovers can escape that night. Basilio enters and is about to expose Almaviva as a fake when he bribes Basilio to feign sickness. Bartolo finally realizes he is being duped and angrily orders the doors guarded. Berta the housekeeper comments on the foolishness of old men wishing to marry young women.

Bartolo tricks Rosina into thinking her "Lindoro" is merely a go-between for Count Almaviva. In her disappointment, she agrees to marry Bartolo. As a storm rages outside Figaro and Almaviva enter, and the Count tells a delighted Rosina his true identity. They express their love while Figaro urges them to get out of the house.

By the time they try to escape, their ladder has disappeared. Basilio enters with a notary. Bribing and threatening Basilio to remain quiet, Figaro instructs the notary to marry Almaviva and Rosina. Bartolo bursts in with the guards, but it is too late. Almaviva reveals his identity to the doctor. Bartolo, relieved that he can keep Rosina's dowry, resigns himself. All wish the couple eternal love and fidelity, while Figaro pronounces the quintessential thesis: "The scoundrels have all the luck in this world."