Outside of Dr. Bartolo's house, a group of musicians, including the wealthy (and disguised) Count Almaviva, serenade Rosina, a beautiful young maiden kept hidden away inside. When Rosina, the ward of Dr. Bartolo, offers no answer to the musicians' serenade, Almaviva pays the musicians and sends them away. Figaro, once employed by Almaviva, arrives singing a song about being the city's factotum. When Figaro comes across Almaviva, Almaviva asks Figaro for help winning over Rosina. Dr. Bartolo leaves the house with plans to marry Rosina himself. Almaviva serenades Rosina once more, telling her his name is Lindoro and that love is all he has to offer. Finally, Figaro suggests that Almaviva disguise himself as a poor drunken soldier ordered to stay, or billeted, with Dr. Bartolo. Almaviva is so delighted with the plan, he pays Figaro generously.
Inside Dr. Bartolo's house, Rosina clearly smitten with Lindoro's song, sings a lovely song ("Una voce poco fa") about the voice she has just heard. She writes a letter to Lindoro, while secretly planning a way to escape from Dr. Bartolo. Moments later, she is joined by Figaro, but the two quickly leave at the sound of footsteps. Dr. Bartolo arrives with Don Basilio, a music tutor. Basilio tells Dr. Bartolo that Almaviva competes with him to win the hand of Rosina, and that Bartolo must slander Almaviva's name. Figaro overhears that Dr. Bartolo plans to marry Rosina the following day, and persuades her to give him the letter she has written to Lindoro so that he can deliver it. Alone with Dr. Bartolo, Rosina is questioned and reminded that Dr. Bartolo is unable to be tricked. Midway through his interrogation, they are interrupted at the sound of vigorous knocking on the door. Berta, Dr. Bartolo's maid, answers the door to find Almaviva as the drunken soldier. She brings him up to Dr. Bartolo. As the two men argue, Almaviva manages to pass a letter along to Rosina, whispering to her that he is Lindoro. Dr. Bartolo sees this and demands Rosina hand him the letter. She complies, but gives him her laundry list instead. Figaro rushes into the room, warning them that their incessant arguing has attracted a crowd, and that authorities are on their way to settle the dispute. Dr. Bartolo, Berta, and Basilio take pleasure in watching the authorities take the disguised Almaviva away from the house. Before he is escorted to jail, they are quickly amazed when he is released without any fuss. Almaviva only had to whisper his identity to them before they complied with letting him go.
Now disguised as the substitute music teacher of Don Basilio, who has been very ill of late, Almaviva arrives to tutor Rosina. Dr. Bartolo is hesitant at first to let him in, but after Almaviva shows him Rosina's letter to Lindoro, Dr. Bartolo allows him to enter. Almaviva tells Dr. Bartolo that he plans to discredit Lindoro, as he thinks he is a servant to and doing the bidding for Count Almaviva. When Almaviva enters the room, Rosina instantly recognizes him as her suitor and the two begin their lesson. Figaro arrives to give Dr. Bartolo his scheduled shaving and takes him to another room, stealing a key to the balcony along the way, leaving the young lovers alone. Don Basilio shows up looking much better, but is quickly turned away when Almaviva bribes him to leave. Almaviva and Rosina discuss their plans to elope, but are overheard by Dr. Bartolo. He immediately kicks Figaro and Almaviva out of the house and sends Rosina to her room. Dr. Bartolo, then, calls for Basilio. Meanwhile, poor Berta can barely keep her mind straight from all the confusion. Dr. Bartolo convinces Rosina that Lindoro is just a henchman of Count Almaviva. Later that evening after a large thunderstorm, Almaviva dressed as his true self arrives with Figaro. The two men climb up to the balcony and unlock Rosina's door. As they begin to abduct Rosina, she initially protests. After Almaviva explains that he has been in disguise as Lindoro the whole time, she quickly gives in and falls into his arms. As they begin to make their way from the house, Basilio arrives with a notary intending to marry Rosina and Dr. Bartolo. After another bribe, Basilio allows the notary to marry Almaviva and Rosina instead. Once the marriage is officiated, Dr. Bartolo arrives. Almaviva makes a deal with Dr. Bartolo that allows Dr. Bartolo to keep the dowry, and Rosina and Almaviva remain together without any objections.