The curtain rises to reveal Susanna, the countesses' maid, grooming herself while Figaro her fiancé is measuring the floor to their new room. Susanna refuses to move to their new abode as a consequence of its proximity to the Counts' chamber. She informs Figaro that the Count has decided to repeal his repudiation of the Right of the Lord just in time for their wedding. At this point Figaro expresses his anger in an ironic aria suggesting that if his Lordship would like to dance then he, Figaro, will provide the accompaniment.

Enter Don Bartolo and Marcellina who are hatching a plot to force Figaro to marry Marcellina as a way of redeeming his debt to her. Bartolo sings his famous Vengeance Aria. They are about to leave the room when Susanna enters via another entrance and the two contenders for Figaro's affection virtually sing a cat duet, at the end of which enters a disconsolate Cherubino.

He has come to enlist Susanna's help in his quest to be reinstated as the Countesses' page boy, a privilege he recently lost as a consequence of being found in Barberina's room. Cherubino sings of his emotions, he loves all women and "he no longer knows who he is or what he does". He hardly has time to complete his aria when the Count is heard at the door. Cherubino hides as the Count enters the room protesting his love for Susanna. The Count is interrupted by Don Basilio, who after teasing Susanna, suggests that the Countess is interested in Cherubino. This is the signal for the Count to reveal himself and also to discover Cherubino hiding behind the chair. Furious at the deception, the Count orders Cherubino to join his regiment. Meanwhile, Figaro has decided on a pre-emptive strike; he enters with all the peasants and servants, praising the Count for renouncing his right of the Lord in favour of his valet. Figaro now discovers Cherubino's plight, and advises him on military life.

The Countess is in her bedroom bemoaning the Count's lack of interest in her. She enlists Figaro and Susanna's aid in an effort to ascertain the Count's infidelity so that she may confront him with it. They send Cherubino into the garden dressed as Susanna. He is instructed to allure the Count and then repulse his advances - giving them an opportunity to denounce his philandering. Cherubino is dressing in Susanna's clothes when the Count enters, forcing him to hide in the dressing room. The Count turns his fury on the Countess who remains impassive and is steadfast in her refutation of any wrong doing. He refuses to send for the servants, preferring to fetch the tools himself for the forceful opening of the dressing room door.

Meanwhile, Susanna returns to the room and freeing Cherubino, who jumps out of the window, takes his place in the dressing room. A very nervous Countess watches as her husband opens the door only to reveal Susanna: the Count is astonished while the Countess is relieved. Figaro interrupts, announcing that all is ready for the wedding feast. The gardener, Antonio, is incensed that someone has jumped into his garden bed just below the window. Figaro pretends to be the culprit. Bartolo and Basilio arrive announcing that Figaro must either marry or pay Marcellina.

Susanna arranges a rendezvous in the garden with the Count, on the understanding that he keep his promise to give her a dowry, but when he overhears his last phrase being repeated to Figaro, he rages with fury.

In his efforts to avoid marriage with Marcellina, Figaro protests that he can not possibly marry whilst his parents are still missing. Further exploration reveals that Marcellina and Basilio are in fact Figaro's parents. Susanna enters just as Figaro embraces Marcellina but is soon reassured that Marcellina is to be her mother in law.

The Countess and Susanna contrive to fake an invitation to the Count, which Susanna delivers during the wedding feast. It is Figaro's turn to be jealous and disgruntled.

Encountering Fiqaro and Marcellina, Barberina inadvertently discloses that Susanna has sent the Count a note. Figaro assumes the worst and swears vengeance on his unfaithful wife. Having swapped clothes and identity with the Countess, Susanna sings of her love for Figaro, who naturally assumes the serenade is meant for the Count. Cherubino enters and, believing the Countess to be Susanna, begins to make passionate love to her. The Count intervenes and failing to realize that the lady is his own wife, begins seducing Susanna most passionately. Figaro informs the Countess (Susanna) of their respective spouse's infidelities, but quickly recognizes Susanna's voice and joins in the deceptive joke. Figaro and Susanna declare ardent love for each other and the Count, tricked by Susanna's disguise is horrified that his wife should be so unfaithful and with his own valet. Believing he has caught his wife in flagrante, the Count calls everyone to the garden where he vents his anger on the Countess. The deception is soon explained and an embarrassed Count begs forgiveness from his wife. She relents and soon all is forgiven and everyone is rejoicing.