In eighteenth-century Naples, the elderly cynic Don Alfonso discusses women with two young officers, Ferrando and Guglielmo. The gallants insist their sweethearts are paragons of virtue.  They accept Alfonso's bet that he can prove the ladies fickle if they do as he says for twenty-four hours.

The sisters Fiordiligi and Dorabella revel in their love for Guglielmo and Ferrando, respectively, showing pictures they carry in their lockets. Alfonso comes in with sad news: the young men have been called to their regiment. They appear, and the five make elaborate farewells As soldiers march by, Ferrando and Guglielmo fall in, lamented in a trio.  Alfonso, alone, delivers one last jeer at women's inconstancy.

The maid, Despina, offers the sisters advice about forgetting old lovers with the help of new ones, but her mistresses resent her capricious approach to love. Dorabella, in fact, is outraged. When they leave, Alfonso comes to bribe Despina to introduce two foreign friends of his to the ladies. The sisters, returning, are scandalized to see the strangers, whom they do not recognize as their lovers, heavily disguised as Albanians. The newcomers declare their admiration for the ladies, but both repulse them, and Fiordiligi likens her fidelity to a rock. The men are thrilled, but Alfonso warns the bet isn't won yet. As Ferrando blissfully reiterates his passion for Dorabella, Despina suggests a plan to Alfonso to win the ladies' sympathy.

Alone in their garden, the sisters lament the absence of their lovers. Suddenly the "Albanians" stagger in, pretending to have poisoned themselves in despair over their rejection. Alfonso and Despina run for a doctor while the ladies begin to waver: pity for the strangers will be their undoing. Despina returns, disguised as a doctor using Dr. Mesmer's invention, the magnet, to draw out the poison, and urging the sisters to nurse the patients as they recover. The men revive but their increased ardor alarms the women, who angrily refuse their demands for a kiss.

Act II

Attending her mistresses, Despina lectures them on their stubbornness and describes how to handle men Dorabella is persuaded there could be no harm in a little flirtation, and surprisingly, Fiordiligi agrees. They decide who will pair off with whom.

The young men have arranged a serenade in the garden. Seeing their wager through, Guglielmo ardently pursues Dorabella while Ferrando woos Fiordiligi who admits he has touched her heart, hoping her absent lover will forgive her. When the men compare notes, Guglielmo is glad to see Fiordiligi standing fast - or so he thinks - but Ferrando is dismayed that Dorabella has given in to Guglielmo, who comments on the waywardness of the fair sex Left alone, Ferrando expresses his love for Dorabella, though he feels betrayed.

Fiordiligi rebukes Dorabella for being fickle, although she admits that in her heart she has succumbed to the stranger. Despina coaxes her to find a way, saying love is a thief and people get robbed every day. Alone, Fiordiligi decides to drag her sister off to join their sweethearts at the front, but Ferrando, pursuing the wager, threatens suicide, and Fiordiligi gives in. Now Guglielmo is furious, but Alfonso counsels forgiveness: that's the way women are, he claims.