In a room located between the apartments of the Count and those of the Countess, Figaro is measuring the floor while Susanna is trying on, and showing off to him, the bonnet she has made for herself. Figaro explains that the room is the most convenient in the castle and that the position will make it easy for her to go to the Countess. Susanna reminds her bridegroom that this makes it easy for the Count to get to her. The Countess rings and Susanna must be off. Figaro leaves and Marcellina enters with Dr. Bartolo. The two of them are hatching a plot which will compel Figaro to marry Marcellina as he has defaulted on a debt that he owes her. Dr. Bartolo, with his legal knowledge, will ensure that there is no escape for the rascal. As he goes out one door, Susanna enters by another and she and Marcellina engage in a little name calling bout ending in Marcellina’s complete discomfiture. Susanna stays behind and is joined by the mercurial Cherubino, who wants to enlist her help in getting the Count to reinstate him as the Countess’s page. Suddenly a voice is heard outside and he has barely time to hide behind a chair before the Count comes in and starts to protest his affections. The Count is followed a moment or two later by Basilio; in the scramble for concealment, Cherubino nips into the chair behind which the Count takes refuge. Basilio teases Susanna with gossip about Cherubino and presses her about the page and the Countess. On hearing this, the Count emerges from his hiding place demanding that this talk be stopped. In the ensuing trio Susanna faints, but revives in time to plead the cause of the unhappy Cherubino, a mere boy she says. Now the Count, while dramatically telling about his discovery at Barbarina’s, at the same time discovers the page in the chair. Only Cherubino’s admission that he has heard what passed between the Count and Susanna stays the penalty that would otherwise be his. Led by Figaro, a band of peasants comes in to sing the Count’s praises, and, at its end, the Count forgives the page and makes him captain of a regiment in Seville, with orders to report there immediately. Figaro speeds him on his way.

We are introduced to the Countess, who regrets the loss of her husband’s love. Susanna admits Figaro, who has a plan. The Count shall be given an assignation with Susanna, whose place shall be taken by Cherubino, and the Count shall be sent an anonymous letter telling him that the Countess has made a rendezvous with an unknown man. Just as Cherubino is dressed, the Count knocks at the door. Cherubino dashes into the Countess’s bedroom and Susanna hides behind a curtain. The Count is suspicious when he hears a noise and finds the door locked. He takes his wife with him as he goes off for tools to force the lock. While he is away Cherubino slips out of hiding and jumps from the window, leaving Susanna to take his place. The finale is begun by the Count in a towering passion. The Countess’s pleading seems to be in vain, but both are amazed when Susanna emerges coolly from the inner room. The Count can do nothing but sue for pardon. Figaro enters to summon his master and mistress to the wedding dance which is about to begin. The Count questions him about the anonymous letter. Figaro denies all knowledge of it, and almost turns the Count’s suspicions when Antonio, the gardener, staggers in, protesting that today they have thrown a man out of the window onto his flower beds. Figaro says it was he who jumped out, but Antonio thinks it looked more like the page. Figaro sticks to his story and the Count asks him about a paper Antonio says was dropped near the flower bed. Figaro racks his brain and in the nick of time the Countess recognizes it and whispers to Figaro that it is the page’s commission. Figaro’s triumph is short lived, as Marcellina comes in, supported by Basilio and Bartolo, to lodge formal complaint before the Count against Figaro for breach of promise. The act comes to an end in pandemonium.

The Count paces to and fro, thinking over the strange events he still cannot explain. Susanna agrees to meet him that night in the garden. The Count is overjoyed. As Susanna leaves the room, she meets Figaro and assures him, just a little too loudly, that he is sure of winning his case against Marcellina. The Count overhears this, and in mounting anger repeats her words. Then, the Countess expresses a situation which can only be resolved with Susanna’s aid, and between them they arrange where Susanna is to meet the Count that evening. The mistress dictates a letter to the Count, but the wedding festivities are about to begin. The village girls come and present their flowers, and Cherubino is discovered by Antonio and the Count. A tense situation is saved by Barbarina, who ingenuously reveals the Count’s advances toward her. While a fandango is danced, the Count secretly opens Susanna’s letter and pricks his finger on the pin, an incident laughingly noted by Figaro. The Count invites all to return for a big celebration that night, and they leave, praising their noble master.

As night is falling over the gardens of the castle, Barbarina is looking for the pin which the Count gave her to return to Susanna. She tells the story to Figaro who is overcome with distress at this apparent indication of his wife’s unfaithfulness. Figaro watches Barbarina hide in one of the arbors (where she is to meet Cherubino), and tells Bartolo and Basilio that they are to stay near at hand to witness the seduction of his wife by the Count. The Countess and Susanna appear, disguised in each other’s clothes. They are joined by Marcellina, who informs them in a whisper that Figaro is listening. Susanna sings an aria, ostensibly to the lover she is waiting for, but in reality knowing full well that Figaro is listening to her. The comedy of mistaken identity begins. Cherubino attempts to flirt with someone he thinks is Susanna, as the real Susanna and Figaro watch unseen. The Count interrupts and starts to make love to his wife in disguise. Figaro does not know about the change of clothes and it is his turn to interrupt. Susanna, still disguised as the Countess, calls to him and he starts to tell her of the Count’s escapade when he recognizes that it is in fact Susanna he is talking to. Figaro makes love to her as if she were the Countess, and he laughs at her attempt to disguise herself from him as much as at her indignation. All is forgiven-Susanna does not mind the joke against herself-and the two combine to make the Count think they are mistress and valet. The ruse succeeds, the Count summons anyone within hearing to witness to the unfaithfulness of his wife, and in succession hauls Cherubino, Barbarina, Marcellina and the supposed culprit from the arbor. All pleading is in vain, until the voice of the Countess herself is heard behind them all. All are perplexed. The Count begs forgiveness, which is lovingly granted by his wife, and the opera ends in general rejoicing.