In the castle of Count Almaviva, Figaro, a servant, and Susan, another servant, are preparing to get married. Susan suspects that the count has put them in a specific bedroom in the castle because it is near to his; she has suspected him of wanting to have an affair for awhile. She and her betrothed, Figaro, try and devise a plan whereby they can trap the count in his lechery.
Doctor Bartholo and Marcelina come in complaining of Figaro's debt to Marcelina. Figaro has made a promise in writing that, if he is unable to repay his debt to Marcelina, he will marry her. Marcelina and Susan engage in a contentious conversation, before getting interrupted by the count's page, Hannibal, who tells Susan that the count fired him for hiding in the countess' bedroom with a servant girl, Agnes. He begs Susan to get the countess to advocate for him, but the count comes in and Hannibal must hide behind a chair.
The count tries to seduce Susan, who refuses, when suddenly Basil enters and the count must hide behind the armchair, while Hannibal jumps onto the armchair underneath one of the countess' gowns. Basil discusses the rumor that the countess is having an affair with Hannibal, which outrages the count, who stands and reveals himself. In the process of railing against Hannibal, the count discovers Hannibal under the gown and dismisses him once and for all.
In the countess' bedroom, Susan tells her mistress all that has happened. Figaro comes in and tells them he started a rumor that the countess is having an affair. The countess and Susan then devise a plan to dress Hannibal up as Susan and have him meet the count that evening. As they begin to dress Hannibal up, the count knocks on the door and Hannibal must hide in the dressing room. The count confronts the countess and tries to open her dressing room, but when he finds it locked, he goes to get an instrument to force it open. In the period when he is out of the room, Hannibal jumps out the window into a flower bed below and Susan takes his place in the dressing room, so that when the count manages to open the door, Susan is standing there.
Before Figaro's trial, Susan and the countess devise a plan to catch the count in his infidelity. Susan tells the count that she will have an affair with him, and Figaro's trial begins. In the course of the trial, Figaro reveals that he was stolen from his birth parents, but he has a scar in the shape of a lobster on his arm. Marcelina instantly recognizes him as her son with Dr. Bartholo, and the forced marriage is forgotten. Susan also gives Figaro the money he owes Marcelina, which the countess gave her.
In the next act, Figaro makes Susan promise not to meet the count, but the countess convinces Susan to help her in a ruse. They write a letter to the count inviting him to meet her in the garden that evening. Figaro marries Susan and Marcelina marries Bartholo. After the ceremony, Figaro sees the count reading the fake letter from Susan and suspects her of having an affair. He becomes deeply angry with his wife and determines to ruin both Susan and the count that evening.
That night in the garden, Figaro delivers a monologue railing against the aristocracy and detailing how hard his life has been. He convenes a group of men to help him apprehend Susan and the Count. The countess and Susan come onstage, dressed as one another, and the countess goes off with the count, who thinks she is Susan. When Susan (dressed as the countess) is left alone, Figaro tries to retaliate by having an affair with "the countess," but soon realizes it is his wife, and laughs heartily at the prank. Later, the count is exposed in his affair and realizes that the woman he thought was Susan was his wife all along.
