Don Alfonso, a philosopher, bets his two young friends Ferrando and Guglielmo that their lovers will prove unfaithful if put to the test. Confidently, they both accept the wager and agree to follow Alfonso’s instructions for 24 hours. Alfonso then tells their lovers, Fiordiligi and Dorabella, that the men must go away to war. They are distraught.  

Next, Alfonso bribes the women’s servant, Despina, to introduce a pair of handsome, foreign ‘strangers’ to them. Fiordiligi and Dorabella are outraged, not realising that they are in fact Ferrando and Guglielmo in disguise! Apparently upset at their rejection, the newcomers pretend to take poison. Fiordiligi and Dorabella are moved — but when Despina, disguised as a doctor, cures the two men, the sisters continue to resist them. 

Despina persuades Fiordiligi and Dorabella to see the strangers once more. Admitting that there is an attraction, they speculate over which of the new men they would choose. Each unwittingly wants the other woman’s real-life love.  

Guglielmo finds Dorabella surprisingly easy to win over. Fiordiligi has more willpower, but her attraction to Ferrando leaves her in an agony of guilt. 

The two men compare notes. Guglielmo gloats, but Ferrando is sickened that he has been betrayed — deceived by Dorabella, and also rejected by Fiordiligi. Alfonso goads him into one last attempt, and this time Fiordiligi’s resolve crumbles. Guglielmo is appalled. Both he and Ferrando admit that Don Alfonso was right: così fan tutte – all women are the same.  

The sisters are ready to marry their new lovers (with Despina disguised as a lawyer) when they hear the sound soldiers returning. Their panic turns to shock when they finally realise that the foreign strangers were Guglielmo and Ferrando all along! Don Alfonso has won his bet.