Figaro hides and listens to the Count serenade Rosina, Dr. Bartolo's ward. Almaviva is in love with Rosina, who he thinks is Dr. Bartolo's daughter. The Count has never actually met Rosina because Dr. Bartolo (hoping to marry her himself and acquire her large dowry) keeps her locked in the house and guarded at all times. Since there is no response to his serenade, Almaviva decides to wait there in hopes of catching a glimpse of her.

Then Figaro appears and soon recognizes the Count, his former employer. When Figaro learns that Almaviva wants to meet Rosina, he offers to help, because as Dr. Bartolo's Barber, he can gain entry to the house - for a suitable reward of course. Figaro and the Count hide as Dr. Bartolo leaves giving his maid, Berta, strict orders to admit no one except Don Basilio, Rosina's music teacher.

Almaviva and Figaro try to think of a way for Almaviva to get into the house. Almaviva, not wanting to be recognized as a Count, decides to use the name " Lindoro". Figaro suggests he force his way into the house pretending to be a soldier billeted to these quarters.

Inside the house, Rosina writes a letter admitting her love for the unknown serenader and her wish to marry him. She says she would be such a fine docile wife if no one stood in her way. Her guardian, Dr. Bartolo, is told that the nobleman, Count Almaviva, has been seen in the neighborhood and suspects that's who has been serenading Rosina.

Figaro warns Rosina of Dr. Bartolo's plan to marry her himself. Rosina is amused and continues to question Figaro about the young man who has been serenading her. Figaro assures her he is a poor, but fine young man, and that he will come to see her if she will write him a letter. She gives the shocked Figaro the letter she has already written.

After Figaro leaves, Dr. Bartolo enters suspiciously and questions Rosina about the ink on her fingers and the missing writing paper. He becomes furious when Rosina thinks up obviously false answers. Suddenly, there is a loud knocking at the door and in comes Almaviva, disguised as a drunken soldier demanding to be housed. Dr. Bartolo protests, trying to refuse entry to the soldier. Rosina discovers, however, that this is "Lindoro". There is great confusion among Rosina, Bartolo, "Lindoro," and Figaro. "Lindoro" finally leaves. Dr. Bartolo wonders if the "soldier" whom he was never able to identify was sent to spy by Almaviva.

Later on the very same day, the Count enters Dr. Bartolo's house again, this time disguised as Don Alonso, a music teacher. He says he is a substitute for Basilio, Rosina's usual music teacher, who is ill. To keep Dr. Bartolo from becoming suspicious, he produces a note written by Rosina to "Lindoro," which he says he found at the inn where Count Almaviva is staying. He suggests this will make Rosina think "Lindoro" is toying with her affections. Bartolo is taken in by his sly story and allows him to give Rosina her singing lesson.

Figaro arrives to shave Dr. Bartolo and manages to pocket the key to the balcony shutters so that the Count may steal Rosina away later on. While Figaro has Dr. Bartolo captive with a full face of shaving cream, Don Alonso, recognized as "Lindoro" by Rosina, proposes marriage and they make plans to elope. Suddenly, Bartolo notices the scheming, and certain that all are trying to cheat him, he chases everyone out. Rosina, Figaro, and "Lindoro" exit laughing, leaving Bartolo in a rage. As she cleans the room, Berta remarks upon how foolish old men are.