Whilst in Madrid, the wealthy Count Almaviva falls head over heels in love with a young woman named Rosine whom he sees for a brief moment and never actually speaks to. After she returns to her home town, Seville, he cannot stop thinking about her and decides to follow her to Seville. Unfortunately this is the extent of his plan as he has no way to meet her. He is also anxious to make sure that she is attracted to him for him, rather than for his wealth, and so decides to disguise himself as a university student called Lindor, who has sufficient means to attract a girl from a nice family but not so much wealth that they would be attracted to him for his money. He then waits outside Rosine's house for her to come outside.
As he waits, he sees a familiar young man by the name of Figaro approaching the house. The two men are well-acquainted and talk for a while. As they do so Dr Bartholo, the owner of the house, and Rosine become visible through one of the upstairs windows. Rosine drops a piece of sheet music from the window, ostensibly by accident, and whilst the doctor comes downstairs to pick it up she signals to Almaviva to pick up the music before the doctor does. He does so and discovers that there is a note for him hidden inside the music, and he quickly removes it before the doctor appears outside. In the note, an intrigued Rosine asks him who he is, and why he is following her. Figaro knows Rosine and explains to the Count that she is the Ward of Dr Bartholo, and that he knows her well because he is both apothecary to the household, and Dr Bartholo's personal barber. In Seville at the time there was a law that stated homes with rooms available must welcome in a military man in need if called upon so Figaro also comes up with a plan; in order to gain access to the house, Almaviva should don a second disguise, this time a drunk military man who needs a place to stay. The doctor will not be able to turn him away, giving him access to Rosine.
Figaro and the Count accidentally eavesdrop on a conversation between the doctor and his good friend Brazile, revealing that they are planning to force Rosine into marriage with her guardian. The Count realizes he needs to work more quickly if he is to persuade Rosine to marry him and so he sings to her from outside the house, explaining that he is just a humble student who has fallen in love with her. She replies from inside the house that she has fallen for him too.
Rosine writes a second note to Almaviva and asks Figaro to deliver it. Of course, he agrees, but as he is readying himself to leave, Bartholo comes in, complaining about him. He suspects that Rosine's attentions are diverting from him to another yet-unidentified suitor and is wondering if this seduction is being carried out by Figaro. He dismisses Rosine from the room so that he can meet with Bazile and continue to plan the wedding she knows nothing about yet. Figaro is still concealed inside a cabinet and consequently overhears almost all of their plans. It has come to Bartholo's attention that the Count has been asking questions all over town about Rosine. The men decide to tell everyone that the Count is a womanizer who cannot be trusted, and the doctor hopes that, should Rosine hear these negative stories, that she will not be interested in any kind of relationship with him. As soon as they leave, Figaro hastens to find Rosine and warn her about the plan they are implementing.
Almaviva, meanwhile, has disguised himself as a military hero in need of accommodation. He asks for lodging but unfortunately the doctor has received an exemption and is not obligated to offer accommodation to passing soldiers. The Count hands Rosine a note surreptitiously whilst the doctor's back is turned. It instructs her to argue with her guardian in order to set their plan to rescue her in motion. She decides that she will do this after her music lesson.
The Count also decides to insert himself into the household again, this time as a substitute music teacher, ostensibly at the request of Bazile whom he claims has asked him to take his place. Rosine quickly realizes that her substitute teacher is her admirer, Lindor. The plot takes on an almost comedic turn, as the Count plays piano whilst Rosine sings, and the doctor falls asleep lulled gently by the music. As soon as he is sleeping the pair begin to kiss passionately, but once the music stops he awakens, and this continues for quite some time. Sensing an impending problem Figaro offers to give the doctor a shave, hoping that this will give the couple some time on their own. As the Count leaves, Bazile arrives and later tells the doctor that he believes the stand-in music teacher was Almaviva. He also expresses his doubts about the doctor's plan to force Rosine to marry him. Unfortunately his reasoning falls on deaf ears; the doctor is resolute and stubborn, and refuses to abandon his scheme. He tells Rosine that the man who accompanied her on the piano that day is the philandering and womanizing Count who is planning to have her kidnapped. Rosine believes him and is both angry and frightened - frightened enough to agree to marry her guardian. When he leaves to procure a judge to marry them immediately she hides in the bedroom and locks the door, to make sure that the kidnappers cannot find her even if they do come to the house.
The house is broken into that evening, but not by fictitious kidnappers; the Count and Figaro plan how to reveal his true identity to Rosine, who is furious with Almaviva and does not hesitate to tell them so. As she is screaming at him she realizes that Figaro is addressing him rather respectfully and asks why. This seems like the appropriate moment for the Count to finally tell her who he is. He also explains that her opinion of him is based on lies and that it is in fact her guardian who intends to force her into a marriage she doesn't want. Rosine finally believes him and when the Judge that Bazile has arranged arrives at the house, the Count wastes no time in asking him to draw up a marriage contact for them both. As the contract is signed the doctor returns home and protests to the judge, demanding that the contract be nullified. The judge refuses and eventually the doctor stops fighting them and resigns himself to the fact that Rosine has married another man, and that his machinations have failed.