Outside Doctor Bartolo’s house
Beneath the balcony of an unknown lady whom he has admired from afar for weeks, Count Almaviva sings a serenade, accompanied by musicians engaged by his servant Fiorello.
The barber Figaro appears. He is a former servant of the Count and knows the object of the Count’s affections: her name is Rosina. As the barber of her guardian, Doctor Bartolo, he has access to his house.
Rosina appears on the balcony and drops a letter before Doctor Bartolo can intervene. In the letter, she asks her admirer to explain his intentions and shares her strong desire to »burst her chains«. Through a song, the Count introduces himself to Rosina as Lindoro, a penniless but honest man – he wants the young woman to choose him out of love and not because of his social standing.
Figaro explains that Doctor Bartolo plans to marry Rosina to obtain her dowry. He suggests that the Count should disguise himself as a soldier with orders to be billeted in Bartolo’s house. To seem harmless he should also pretend to be drunk.
Inside Doctor Bartolo’s house
Rosina is determined to employ all her ingenuity and wilfulness to win Lindoro, despite Bartolo’s opposition. Doctor Bartolo suspects that Rosina and Figaro are hatching a plot. He questions first Rosina, then the housekeeper Berta and the servant Ambrogio, but he is unable to glean any information.
Rosina’s music teacher Don Basilio tells Doctor Bartolo that Rosina’s secret admirer, Count Almaviva, has been seen in the city. Doctor Bartolo resolves to marry Rosina the very next day. Don Basilio proposes to discredit the Count by spreading rumours about him.
Figaro has been listening to the two of them and tells Rosina about Bartolo's plans. Rosina is interested above all in Lindoro, whom she has seen with Figaro. Figaro claims that Lindoro is his cousin and is very much in love with Rosina. Figaro asks her to write a letter to Lindoro – but it transpires that Rosina has already written one.
Bartolo finds out about Rosina’s secret correspondence and declares that she will be guarded even more closely.
Disguised as a drunken soldier, the Count enters Bartolo's house. His attempt to slip Rosina a note leads to an uproar that attracts the attention of the Officer of the Watch. He arrests the alleged soldier as a troublemaker. However, when Almaviva covertly identifies himself as a count, to the astonishment of all present the officer immediately retracts his order.
Inside Doctor Bartolo’s house
The Count reappears. He is now disguised as a music teacher, Don Alonso, and explains that he is standing in for Don Basilio, who has been taken ill. He tells the suspicious Bartolo that he was lodging at the same inn as Count Almaviva. He shows him Rosina’s note, saying he found it at the inn. He says he plans to use it to persuade Rosina that she has been duped. Bartolo consents and calls Rosina for her singing lesson. Rosina recognizes Don Alonso as her admirer Lindoro. Figaro arrives to shave Doctor Bartolo. Taking a hint from Rosina, he manages to steal the key to the balcony door.
The sudden appearance of Don Basilio threatens to expose the Count. He surreptitiously slips Don Basilio some money, and all join forces to usher the music teacher out of the house.
Figaro tries to divert Doctor Bartolo’s attention away from the two lovers, but the doctor hears a careless word spoken by the Count and sees through the deception. The Count and Figaro take to their heels.
Don Basilio tells Bartolo of his suspicion that Don Alonso was sent by Count Alma-viva. Doctor Bartolo now wants to expedite his marriage to Rosina even more urgently and sends Don Basilio to fetch the notary.
Bartolo shows Rosina her note as proof that Don Alonso and Figaro were planning to foist her off on Count Almaviva. Rosina, stunned by this revelation, declares that she is ready to marry Bartolo. She also reveals that Figaro and her admirer were planning to enter the house at midnight to kidnap her.
When Figaro and the Count arrive, Rosina rejects the supposed matchmaker Lindoro, whose deception she has seen through. When Lindoro identifies himself as Count Almaviva and confirms the seriousness of his intentions, Rosina is over-joyed.
Don Basilio appears with the notary. Figaro explains that those present are his niece and Count Almaviva, whose wedding the notary is to conduct that very evening in Figaro’s house. The notary has brought the marriage contract with him. The Count gives the protesting Don Basilio a choice between a precious ring or two bullets in his head. Don Basilio chooses the ring and signs the marriage contract as a witness. Doctor Bartolo has fetched several soldiers and demands that they arrest the burglars. When Count Almaviva reveals his true identity to them, shows them the signed marriage contract and gives Bartolo Rosina’s dowry, Bartolo finally accepts defeat.