Act I

Don Alfonso, a philosopher, ironises on the subject of women’s constancy, causing Ferrando and Guglielmo, two young officers, to declare their unwavering confidence in the faithfulness of their respective brides-to-be, Dorabella and Fiordiligi. Don Alfonso wagers a hundred sequins that the objects of the young girls’ affections can be changed in the course of a day. On their honour as soldiers, the young men undertake to obey Don Alfonso in everything for the next twenty-four hours. Convinced of their victory, they discuss what they will buy with their winnings: Ferrando, a serenade to his beloved, Guglielmo, a banquet. In a garden on the coast, Fiordiligi and Dorabella lovingly contemplate the lockets which enclose their fiancés’ portraits. Don Alfonso, feigning deep sorrow, announces that their lovers have been mobilised and must leave immediately. The two officers come to bid farewell to their fiancées whose despair prompts Don Alfonso to further irony. Despina, the chamber maid, bemoans her lot as a servant. Dorabella’s grandiloquent grief at first worries Despina, but on learning its cause, she suggests her mistresses profit from the absence of their fiancés to amuse themselves. Incensed, Fiordiligi and Dorabella depart. Don Alfonso asks Despina to help him in his schemes in return for some reward, but without revealing to her the full extent of the hoax. He introduces Ferrando and Guglielmo, disguised as Albanians. On returning, Fiordiligi and Dorabella find Despina in the company of the two “strangers”. They express their indignation. Don Alfonso, pretending he has just arrived, introduces the “Albanians” as long-time friends. Guglielmo boasts about their admirable physiques, which provokes the departure of the outraged sisters. Ferrando and Guglielmo consider that they have won, but in Don Alfonso’s view everything is still to play for. In the garden, the sisters lament. The two “Albanians” feign suicide under their eyes by swallowing so-called poison. Don Alfonso calls for a doctor, a disciple of Mesmer, who is none other than Despina in disguise. The two men are miraculously revived and resume their vehement wooing of the young girls.

Act II

Despina gives the sisters a lesson on the behaviour to adopt towards men. Fiordiligi and Dorabella agree to amuse themselves in the company of the “Albanians”, each choosing the fiancé of the other. The two men serenade their sweethearts. Dorabella does not hold out long against Guglielmo’s ardent declarations and gives him the locket containing Ferrando’s portrait. Fiordiligi, though, rejects Ferrando’s advances despite the growing confusion she feels. Ferrando tells his friend of his failure, but is forced to confront Dorabella’s betrayal. Guglielmo declares he has won his half of the wager. Don Alfonso reminds them of the terms of their agreement; he is not beaten yet. Fiordiligi, who is less and less sure of her feelings, decides to join her fiancé with the troops, but Ferrando intervenes. This time she is unable to resist his passionate professions of love and succumbs. Don Alfonso is triumphant: così fan tutte (women are all alike!). He soothes the anger of the two officers and suggests that they marry their sweethearts that very evening. The wedding is being prepared. Despina, disguised as a notary, draws up the fake marriage contracts. Just as they are being signed, a military march signals the return of the fiancés. The young girls panic. The “Albanians” pretend to hide and return as their true selves. They affect surprise on discovering the bogus notary and marriage contracts and demand an explanation from their dismayed fiancées. Don Alfonso then reveals the deception and asks the four young people to let themselves be guided by reason.