In Madrid, Count Almaviva encounters a woman whose beauty fascinates him so much that he follows her to Seville, where her guardian, Doctor Bartolo, has his medical practice. Night and day, the Count serenades beneath Rosina's windows, but without success. Unexpectedly, his former servant Figaro appears. He has set himself up as a barber and has become indispensable as factotum in Bartolo's house. He explains to Almaviva that the guardian wants to take possession of Rosina's inheritance by marrying her soon, and that is why he keeps a close watch on her. When Rosina appears on the balcony, Figaro encourages the count to sing another serenade. Almaviva presents himself as a poor but true admirer of her. After he has been assured of a rich reward, Figaro works out a plan on how to bring the two together. The count is to dress up as a drunken soldier to allay Bartolo's suspicions. Equipped with the appropriate army notification, the Count is to insist on being accommodated in the house.
Rosina has written a letter and hopes that Figaro will give it to him. Bartolo who is suspicious of the busy barber, prevents it. Rosina's music master, the scheming Don Basilio, informs Bartolo about the Count Almaviva being in pursuit of his ward. While Basilio urges him to deal with the matter by slandering his rival, Bartolo hurries arrangements along for a quick wedding. Figaro, who has secretly heard this conversation, warns Rosina and asks her for a sign of her affection for Lindoro. Bartolo's suspicions set him on the trail of the letter, but Rosina boldly refutes all his arguments. Almaviva noisily enters the house as a drunken soldier. He succeeds in revealing himself to Rosina as Lindoro and secretly hands her a note. But he does not succeed in his real aim of being accommodated, as Bartolo as physician is released from the obligation of providing military quarters. The quarrel with the guardian ends in a tumult which brings the guards to the spot. The count escapes arrest only by showing a document.