Comedic twists and turns provide fertile soil for the all-too-human passions that are triggered by a »test of fidelity«, resulting in a veritable mêlée of conflicting emotions. Ferrando and Guglielmo, firmly convinced of the undying love of their fiancées Dorabella and Fiordiligi, agree to take part in a bet with Don Alfonso, who intends to prove the infidelity of the two women. According to his instructions the two young men, while in disguise, succeed in wooing Dorabella and Fiordiligi, who, after initial resistance, unreservedly pledge themselves to the »wrong« husband-to-be. A potential solution to this thicket of emotions only appears at the very end of the piece. Emperor Joseph II is believed to have commissioned the opera, which received its premiered in Vienna in 1790. The libretto by Lorenzo da Ponte and Mozart’s brilliant score serve to spellbind the audience, taking them on a journey from light comedy to high drama.
By means of an amusing masquerade the work brings to light passions which are at the very core of our existence; passions which none of the characters are able to resist.
Ferrando and Guglielmo are firmly convinced of the love of their fiancées, Dorabella and Fiordiligi respectively. So convinced that they casually take up the wager of the “Philosopher”, Don Alfonso, that he can prove, within a single day, that their two ladies are unfaithful. The ladies are to be put to the test in strict compliance with his rules and the plot is to be kept in total secrecy.
While the two sisters, Dorabella and Fiordiligi, are waiting for their loved ones, Don Alfonso arrives instead and gives them the dreadful news that their men have to go off to war and depart immediately. The two pairs take their leave of each other.
The two sisters are left inconsolable. Not even their maid, Despina, can cheer them up. She tries to win them over to the idea that the absence of their fiancés will give them the chance of a dalliance with other men.
Despina is easily persuaded by Don Alfonso to give him active support in his wager with Ferrando and Guglielmo. These two now appear, disguised as “strangers”, and when they make advances to Fiordiligi and Dorabella, each to the other's betrothed, they are briskly repulsed. They pretend to make an attempt on their own lives, in despair at unrequited love – a “doctor” is called, to restore their apparent corpses to life – but this stratagem is equally unsuccessful in winning the two ladies round. Still, there are slight signs that Dorabella and Fiordiligi are interested in the two strangers.
Despina puts the case for the strangers to the two sisters. The intrigue starts to show clear signs of success, for after some coy hesitation Dorabella yields to Guglielmo's charms. Dorabella seems to remain steadfast, and even plans to follow her fiancé onto the field of battle. But in the end she too abandons her principles and stops rejecting Ferrando's advances.
Appalled by the discovery that their fiancées are evidently capable of unfaithfulness, Ferrando and Guglielmo go back to Don Alfonso. It is clear that he has won the wager, and he taunts them with his maxim on the fickleness of women: “Così fan tutte” - they're all like that!
Soon Don Alfonso is busy planning a double wedding for the two new pairs of lovers. A “notary” is ordered, and with the ceremony already under way the real fiancés appear, Guglielmo and Ferrando. The time has come for Don Alfonso to reveal all the details of his stratagem.
The original pairs are now reunited.