Così fan tutte

Act One
Two young couples are spending their summer on a Mediterranean beach, the sisters Fiordiligi and Dorabella and their fiancés Guglielmo and Ferrando. An older couple is also on the scene, Don Alfonso and Despina.
Alfonso proposes a wager to Guglielmo and Ferrando: Alfonso bets 100 sequins that he could prove in just one day that Fiordiligi and Dorabella would be unfaithful. But they would have to follow all of Alfonso’s instructions during those 24 hours and promise not to reveal the plan to the sisters. Guglielmo and Ferrando agree to the plan because they are so confident of their lovers’ fidelity.
Fiordiligi and Dorabella are happy to have two such splendid, strapping fiancés. Alfonso brings the news that Guglielmo and Ferrando have been suddenly summoned to serve military duty. He says there is nothing that can be done to change this, and so the couples must bid one another farewell, but they swear that they will remain true to one another. The young men leave while those remaining wish them good travels and a safe return. Alfonso will soon again meet Guglielmo and Ferrando to give them new instructions.
Despina encounters Dorabella and Fiordiligi in utter despair: without their betrothed life seems meaningless. They reject Despina’s advice not to take the temporary separation too hard. While they wallow in heartbreak, Despina makes clear that she has no faith in the fidelity of men or soldiers.
Alfonso lets Despina in on his plans. In just a moment, two strangers will appear that seek to court Fiordiligi and Dorabella. In costume, Guglielmo and Ferrando now enter, declaring their respect and love to the two women. Alfonso pretends he is a good friend of the two men and sings their praises to the women. Fiordiligi insists that she and her sister refuse to give in to their wooing and violate their fidelity to their betrothed.
Guglielmo emphasized the advantages that he and his friend have to offer, saying that, with them, Fiordiligi and Dorabella will certainly be happy and satisfied. The women reject their advances.
But when they are alone again with Alfonso, Guglielmo and Ferrando are in jovial spirits and think they have already won the bet. Alfonso says not to count your chickens before they hatch. Together with Despina, he plans to convince Fiordiligi and Dorabella to be unfaithful to their partners. They are still worried about the absence of their betrothed. Still in costume as the two strangers, Guglielmo and Ferrando undertake a fake suicide attempt to win over the compassion of Fiordiligi and Dorabella. Despina, disguised as a doctor, pretends to prevent their death by poisoning with a strange healing method based on magnetism.
Guglielmo and Ferrando are now even more aflame than before, and Fiordiligi and Dorabella find it increasingly difficult to resist the men’s advances.

(Intermission)

Act Two
Despina tells Fiordiligi and Dorabella that there is nothing wrong with giving in to two strangers who are so desperately in love with them. And in fact, they are slowly beginning to like the idea, but Fiordiligi prefers Ferrando and Dorabella Guglielmo, the respective other from the original couples.
The two young men sing a serenade to their pretend beloved. Alfonso encourages the women once again not to be standoffish and Despina supports him in this.
Guglielmo wins over Dorabella’s affections, and after a brief time they soon give their hearts to one another. But Fiordiligi continues to reject Ferrando, saying she cannot be unfaithful to her betrothed, her thoughts are with him.
Ferrando and Guglielmo report to each other what has happened. While Fiordiligi’s faithfulness, to Guglielmo’s satisfaction, seems unbroken, Ferrrando has to accept that Dorabella is clearly not quite as serious in her feelings. Guglielmo sees that as women’s true character, Ferrando in turn fears the mockery of Alfonso, but still feels attracted to Dorabella.
Alfonso is ready to continue the game, for Guglielmo and Ferrando have to follow his wishes until the morning. Dorabella admits to being in love with the stranger that she has chosen. Fiordiligi reveals that she too is in love, not just with her betrothed. Since the odds that their fiancés will actually return are not so great, she suggests that it would surely be better to go with the two strangers, since they have won over their feelings. Dorabella tries to convince her sister to agree to this decision.
But Fiordiligi refuses, instead she wants to dress in men’s clothes and rush off to her betrothed to restore her supposed lost honor. Ferrando, however, once again presses on with declarations of love. Ultimately, Fiordiligi can no longer resist. Guglielmo is furious: he now also had to acknowledge his own fiancée’s infidelity. The two young men are now ready to leave their former respective partners in light of what has happened. But Alfonso promises to fix everything: he excuses the actions of the women, ending by saying:
“Così fan tutte – That’s how they all do it”.
Despina reports that the marriage of the two new “fake” couples is prepared. They are festively received, and Despina, dressed as a notary, reads the marriage contract. In the moment that the four parties want to sign, the return of the actual betrothed is announced. Fiordiligi and Dorabella are afraid of what will happen now. Guglielmo and Ferrando, now having returned to their actual identity and form, greet their betrothed after just a day of absence. The sisters are overwhelmed by the situation. Despina admits to having dressed as a notary and playing along in the masquerade. When Guglielmo and Ferrando discover the marriage contracts and accuse the two women of betrayal, the sisters are forced to admit their unfaithfulness – severe punishment would only be just. But the men now reveal themselves as the strangers who had pursed them a whole day long hat that they had won over. Alfonso tries to reconcile the couples, saying that all the deceptions and disappointments took place just to make everyone happier in the end.