Don Bartolo’s house. Rosina, wily and resourceful, has decided to assert herself: she has written a letter to her young suitor and intends having it delivered by Figaro. The barber arrives shortly after, but is immediately compelled to hide. In fact, Don Bartolo and Don Basilio, Rosina’s singing teacher, enter. The guardian, having sensed the existence of a suitor, wants to speed up the marriage procedure. Basilio, a hypocritical shady operator and swindler, has discovered that Almaviva, Rosina’s suitor, has arrived in town, and suggests Don Bartolo that slander would be the best way to put him out of the running.
The pair leave to prepare the marriage contract, but Figaro has overheard everything. He tells Rosina of the plot and, after having confirmed that Lindoro, whom he passes off as his cousin, is in love with her, tells her that the young man will try to enter the house. He therefore suggests she gives her suitor a small sign of encouragement. No sooner said than done: Rosina gives the barber the letter she had already prepared.
Figaro leaves and Don Bartolo enters. Nothing slips the suspicious old man: he notices that Rosina has used pen and paper and reprimands her. But then someone knocks at the door: the Count enters, disguised as a soldier and pretending to be drunk. Don Bartolo attempts unsuccessfully to send him away. When Rosina arrives, Almaviva reveals his identity and tries to pass her a note, but the tutor notices and, just in time, the girl manages to replace it with the laundry list. At the climax of the confusion, someone else knocks at the door: the guards. The Count secretly reveals his true identity to the officer who orders his arrest and, to everybody’s astonishment, is released. Nobody understands what is happening.