I. Figaro is happily measuring the space where the bridal bed will fit while Susanna is trying on her wedding bonnet in front of the mirror (in the present day, a more traditional French floral wreath or a modern veil are often substituted, often in combination with a bonnet, so as to accommodate what Susanna happily describes as her wedding "capellino"). (Duet: Cinque, dieci, venti, trenta – "Five, ten, twenty, thirty"). Figaro is quite pleased with their new room; Susanna far less so. She is bothered by its proximity to the Count's chambers: it seems he has been making advances toward her and plans on exercising his "droit de seigneur", the purported feudal right of a lord to bed a servant girl on her wedding night before her husband can sleep with her. The Count had the right abolished when he married Rosina, but he now wants to reinstate it. Figaro is livid and plans to outwit the Count (Cavatina: Se vuol ballare, signor contino – "If you want to dance, sir Count").
Figaro departs, and Dr. Bartolo arrives with Marcellina, his old housekeeper. Marcellina has hired Bartolo as her counsel, since Figaro had once promised to marry her if he should default on a loan she had made to him, and she intends to enforce that promise. Bartolo, still irked at Figaro for having facilitated the union of the Count and Rosina (in The Barber of Seville), promises, in comical lawyer-speak, to help Marcellina (aria: La vendetta – "Vengeance").
Bartolo departs, Susanna returns, and Marcellina and Susanna share an exchange of very politely delivered sarcastic insults (duet: Via, resti servita, madama brillante – "After you, brilliant madam"). Susanna triumphs in the exchange by congratulating her rival on her impressive age. The older woman departs in a fury.
Cherubino then arrives and, after describing his emerging infatuation with all women and particularly with his "beautiful godmother" the Countess (aria: Non so più cosa son – "I don't know anymore what I am"), asks for Susanna's aid with the Count. It seems the Count is angry with Cherubino's amorous ways, having discovered him with the gardener's daughter, Barbarina, and plans to punish him. Cherubino wants Susanna to ask the Countess to intercede on his behalf. When the Count appears, Cherubino hides behind a chair, not wanting to be seen alone with Susanna. The Count uses the opportunity of finding Susanna alone to personally step up his demands for favours from her, including financial inducements to sell herself to him. As Basilio, the slimy music teacher, arrives, the Count, not wanting to be caught alone with Susanna, hides behind the chair. Cherubino leaves that hiding place just in time, and jumps onto the chair while Susanna scrambles to cover him with a dress. Now the Count is behind the chair and Cherubino is on the chair covered by a dress.
When Basilio starts to gossip about Cherubino's obvious attraction to the Countess, the Count angrily leaps from his hiding place. Lifting the dress from the chair he finds Cherubino. The young man is only saved from punishment by the entrance of the peasants of the Count's estate, this entrance being a preemptive attempt by Figaro to commit the Count to a formal gesture symbolizing the promise of Susanna's entering into the marriage unsullied. The Count evades Figaro's plan by postponing the gesture. The Count says that he forgives Cherubino, but he dispatches him to Seville for army duty. Figaro gives Cherubino advice about his new, harsh, military life from which women will be totally excluded (aria: Non più andrai – "No more gallivanting").
II. The Countess laments her husband's infidelity. (aria: Porgi, amor, qualche ristoro – "Grant, love, some comfort"). Susanna comes in to prepare the Countess for the day. She responds to the Countess's questions by telling her that the Count is not trying to "seduce" her, he is merely offering her a monetary contract in return for her affection. Figaro enters and explains his plan to distract the Count with anonymous letters warning him of adulterers. He has already sent one to the Count (via Basilio) that indicates the Countess has a rendezvous that evening of her own. They hope that the Count will be too busy looking for imaginary adulterers to interfere with Figaro's and Susanna's wedding. Figaro additionally advises the Countess to keep Cherubino around by dressing him as a girl. Figaro leaves.
Cherubino arrives, eager to be dressed up by the Countess and Susanna. Susanna urges him to sing the song he wrote for the Countess (aria: Voi che sapete che cosa è amor – "You ladies who know what love is, is it what I'm suffering from?"). After the song, they proceed to attire him in women's clothes (aria of Susanna: Venite, inginocchiatevi! – "Come, kneel down before me"). At this time, the Countess sees Cherubino's military commission, and notes that the Count was in such a hurry that he forgot to seal it with his signet ring (which was necessary to make it an official document). Susanna returns to her room for some clothing in which to dress Cherubino. While the Countess and Cherubino are waiting for Susanna to come back, they suddenly hear the Count arriving. Cherubino hides in the closet. The Count demands to be allowed into the room and the Countess reluctantly unlocks the door. The Count enters and hears a noise from the closet. He tries to open it, but it is locked. The Countess tells him it is only Susanna, trying on her wedding dress. The Count shouts for her to identify herself by her voice, but the Countess orders her to be silent. At this moment, Susanna re-enters unobserved, quickly realises what's going on, and hides behind a couch (Trio: Susanna, or via sortite! – "Susanna, come out!"). Furious and suspicious, the Count leaves, with the Countess, in search of tools to force the closet door open. As they leave, he locks all the bedroom doors to prevent the intruder from escaping. Cherubino and Susanna emerge from their hiding places, and Cherubino escapes by jumping through the window into the garden. Susanna then takes his place in the closet, vowing to make the Count look foolish. (duet: Aprite, presto, aprite – "Open the door, quickly!").
The Count and Countess return. The Countess desperately admits that Cherubino is hidden in the closet. The raging Count draws his sword, promising to kill Cherubino on the spot, but when the door is opened, they both find to their astonishment only Susanna. The Count demands an explanation; the Countess tells him it is a practical joke, to test his trust in her. Shamed by his jealousy, the Count begs for forgiveness. When the Count presses about the anonymous letter, Susanna and the Countess reveal that the letter was written by Figaro, and then delivered through Basilio. Figaro then arrives and tries to start the wedding festivities, but the Count berates him with questions about the anonymous note. Just as the Count is starting to run out of questions, Antonio the gardener arrives, complaining that a man has jumped out of the window and broken his flowerpots. The Count immediately realizes that the jumping fugitive was Cherubino, but Figaro claims it was he himself who jumped out the window, and fakes a foot-injury. Antonio brings forward a paper which, he says, was dropped by the escaping man. The Count orders Figaro to prove he was the jumper by identifying the paper (which is, in fact, Cherubino's appointment to the army). Figaro is able to do this because of the cunning teamwork of the two women. His victory is, however, short-lived; Marcellina, Bartolo, and Basilio enter, bringing charges against Figaro and demanding that he honor his contract to marry Marcellina. The Count happily postpones the wedding in order to investigate the charge.
III. The Count mulls over the confusing situation. At the urging of the Countess, Susanna enters and gives a false promise to meet the Count later that night in the garden (duet: Crudel, perché finora – "Cruel girl, why did you make me wait so long"). As Susanna leaves, the Count overhears her telling Figaro that he has already won the case. Realizing that he is being tricked (aria: Hai già vinta la causa ... Vedrò mentr'io sospiro – "You've already won the case?" ... "Shall I, while sighing, see"), he resolves to make Figaro pay by forcing him to marry Marcellina.
Figaro's trial follows, and the judgment is that Figaro must marry Marcellina. Figaro argues that he cannot get married without his parents' permission, and that he does not know who his parents are, because he was stolen from them when he was a baby. The ensuing discussion reveals that Figaro is the long-lost illegitimate son Rafaello of Bartolo and Marcellina. A touching scene of reconciliation occurs. During the celebrations, Susanna enters with a payment to release Figaro from his debt to Marcellina. Seeing Figaro and Marcellina in celebration together, Susanna mistakenly believes that Figaro now prefers Marcellina over her. She has a tantrum and slaps Figaro's face. Figaro explains, and Susanna, realizing her mistake, joins the celebration. Bartolo, overcome with emotion, agrees to marry Marcellina that evening in a double wedding (sextet: Riconosci in questo amplesso una madre – "Recognize a mother in this hug").
All leave, and the Countess, alone, ponders the loss of her happiness (aria: Dove sono i bei momenti – "Where are they, the beautiful moments"). Susanna enters and updates her regarding the plan to trap the Count. The Countess dictates a love letter for Susanna to give to the Count, which suggests that he meet her that night, "under the pines". The letter instructs the Count to return the pin which fastens the letter. (duet: Sull'aria...che soave zeffiretto – "On the breeze… What a gentle little Zephyr").
A chorus of young peasants, among them Cherubino disguised as a girl, arrives to serenade the Countess. The Count arrives with Antonio, and, discovering the page, is enraged. His anger is quickly dispelled by Barbarina (a peasant girl, Antonio's daughter), who publicly recalls that he had once offered to give her anything she wants, and asks for Cherubino's hand in marriage. Thoroughly embarrassed, the Count allows Cherubino to stay.
The act closes with the double wedding, during the course of which Susanna delivers her letter to the Count. Figaro watches the Count prick his finger on the pin, and laughs, unaware that the love-note is from Susanna herself. As the curtain drops, the two newlywed couples rejoice.
IV. Following the directions in the letter, the Count has sent the pin back to Susanna, giving it to Barbarina. Unfortunately, Barbarina has lost it (aria: L'ho perduta, me meschina – "I lost it, poor me"). Figaro and Marcellina see Barbarina, and Figaro asks her what she is doing. When he hears the pin is Susanna's, he is overcome with jealousy, especially as he recognises the pin to be the one that fastened the letter to the Count. Thinking that Susanna is meeting the Count behind his back, Figaro complains to his mother, and swears to be avenged on the Count and Susanna, and on all unfaithful wives. Marcellina urges caution, but Figaro will not listen. Figaro rushes off, and Marcellina resolves to inform Susanna of Figaro's intentions. Marcellina sings of how the wild beasts get along with each other, but rational humans can't. (aria: Il capro e la capretta – "The billy-goat and the she-goat"). (This aria and Basilio's ensuing aria are usually omitted from performances due to their relative unimportance, both musically and dramatically; however some recordings include them.)
Actuated by jealousy, Figaro tells Bartolo and Basilio to come to his aid when he gives the signal. Basilio comments on Figaro's foolishness and claims he was once as frivoulous as Figaro was. He tells a tale of how he was given common sense by "Donna Flemma" and ever since he has been aware of the wiles of women (aria: In quegli anni – "In youthful years"). They exit, leaving Figaro alone. Figaro muses on the inconstancy of women (aria: Aprite un po' quegli occhi – "Open your eyes"). Susanna and the Countess arrive, dressed in each other's clothes. Marcellina is with them, having informed Susanna of Figaro's suspicions and plans. After they discuss the plan, Marcellina and the Countess leave, and Susanna teases Figaro by singing a love song to her beloved within Figaro's hearing (aria: Deh, vieni, non tardar – "Oh come, don't delay"). Figaro is hiding behind a bush and, thinking the song is for the Count, becomes increasingly jealous.
The Countess arrives in Susanna's dress. Cherubino shows up and starts teasing "Susanna" (really the Countess), endangering the plan. Fortunately, the Count gets rid of him by striking out in the dark. His punch actually ends up hitting Figaro, but the point is made and Cherubino runs off.
The Count now begins making earnest love to "Susanna" (really the Countess), and gives her a jewelled ring. They go offstage together, where the Countess dodges him, hiding in the dark. Onstage, meanwhile, the real Susanna enters, wearing the Countess' clothes. Figaro mistakes her for the Countess, and starts to tell her of the Count's intentions, but he suddenly recognizes his bride in disguise. He plays along with the joke by pretending to be in love with "my lady", and inviting her to make love right then and there. Susanna, fooled, loses her temper and slaps him many times. Figaro finally lets on that he has recognized Susanna's voice, and they make peace, resolving to conclude the comedy together.
The Count, unable to find "Susanna", enters frustrated. Figaro gets his attention by loudly declaring his love for "the Countess" (really Susanna). The enraged Count calls for his people and for weapons: his servant is seducing his wife. Bartolo, Basilio and Antonio enter with torches as, one by one, the Count drags out Cherubino, Barbarina, Marcellina and the "Countess" from behind the pavilion.
All beg him to forgive Figaro and the "Countess", but he loudly refuses, repeating "no" at the top of his voice, until finally the real Countess re-enters and reveals her true identity. The Count, seeing the ring he had given her, realizes that the supposed Susanna he was trying to seduce, was actually his wife. Ashamed and remorseful, he kneels and pleads for forgiveness himself (Contessa, perdono – "Countess, forgive me"). The Countess, more kind than he (Piú docile io sono – "I am more kind"), forgives her husband and all are contented. They celebrate as the opera ends, vowing to party all night.