Two young military officers, Ferrando and Guglielmo, boast to Don Alfonso that their fiancées, Dorabella and Fiordiligi, are irreproachable in their steadfastness and virtue. The cynical bachelor Alfonso says that he is ready to wager that, given the right circumstances, both girls will forget their promises and take new lovers. The two young men confidently accept the bet and agree to follow Alfonso's instructions for the next twenty-four hours. Meanwhile, Dorabella and Fiordiligi gaze at the portraits of their lovers and pour out their heartfelt emotions. Alfonso enters, announcing that the two officers are coming to say 'goodbye', because they have suddenly been called to war. This is a ruse invented by Alfonso to test the girls' fidelity. The girls and Don Alfonso bid the soldiers a fond farewell. The maid Despina tries to console Fiordiligi and Dorabella. Dorabella laments her despair in the great aria Smanie implacabili (Unending Longing). Despina tells them not to expect their sweethearts to be faithful - "Fidelity in a man - in a soldier," she tells them, "is just sentimental drivel." The sisters leave, and Don Alfonso asks Despina to help him in his ruse. She agrees, for a price, to introduce two new suitors to Fiordiligi and Dorabella. They are in fact Ferrando and Guglielmo in disguise. The reaction of the sisters is to immediately throw them out, and in the aria Come Scoglio (Like a Rock), Fiordiligi says her fidelity is unwavering. With this stern reception, Ferrando and Guglielmo think they have won and demand payment from Alfonso, who reminds them the day is not over and he who laughs last, laughs best. As the sisters lament the loss of their fiancées, the boys, still in disguise, enter with Alfonso. Guglielmo and Ferrando say they will take poison if the girls do not pay attention to them - and so they drink and fall to the ground. Despina runs off to supposedly get a doctor and the girls, now left alone with their new suitors, are moved by their desperate attempt at suicide for love. Despina enters, disguised as the doctor, and cures the boys who immediately resume their declarations of love as the Act I curtain falls.
Dorabella and Fiordiligi are persuaded by Despina to meet their new suitors that evening. In the delightful aria Una donna a quindici anni (What Every Girl Fifteen or Older Needs to Know), Despina gives them a little lesson - it is a woman's job to manage men, using all their charms to be the queen of their kingdom. That evening, Don Alfonso and Despina arrange that the suitors be paired off - Guglielmo with his friend's fiancée Dorabella and Ferrando with Fiordiligi. Dorabella begins to encourage Guglielmo while Fiordiligi remains steadfast. When the boys compare notes, Ferrando despairs that his Dorabella could be so fickle, while Guglielmo rejoices that his Fiordiligi has remained constant. Fiordiligi declares that she is going off to join her fiancée. Dressed as a soldier, she is about to leave when Ferrando, still in disguise, enters. His arguments this time are successful, and she falls into his arms. It is now Guglielmo's turn to despair which he does in this great aria in which he excoriates all women. Don Alfonso claims victory. All that's left is to prepare for the wedding. The wedding banquet is prepared and the farce reaches its climax. The two couples - Guglielmo and Dorabella, Ferrando and Fiordiligi sign wedding contracts before a notary, who is really Despina in disguise. Just then, military drums are heard announcing the return of the army. The boys quickly hide, change back to their original clothes and return as themselves. Don Alfonso hands over the wedding contracts to the young men, who refuse to listen to their sweetheart's protests. The boys draw their swords, rush off to pursue their fake rivals, and re-enter with half of their disguises on. The farce is revealed to the sisters. Don Alfonso calms the girls down and the couples - but which ones? - are united.