Don Alfonso bets the two young officers that their fiancées — the sisters Fiordiligi and Dorabella — will not remain faithful if put to the test. The plan: the two pretend to leave for the war, then return in disguise as Albanian strangers and court each other's fiancée. Fiordiligi and Dorabella weep desperately at the departure of their loves. Don Alfonso smiles.

The two return unrecognisable in moustaches and exotic costumes. They pretend to be struck at first sight by the two women. Dorabella and Fiordiligi are indignant — how dare they? Guglielmo and Ferrando congratulate each other: their women are perfect. Don Alfonso waits.

Despina persuades the two sisters that a little diversion does no harm. Dorabella yields first, to Guglielmo — her sister's fiancé. Fiordiligi holds out longer, with passionate speeches about fidelity, but eventually she too yields to Ferrando. The two officers are shaken: each has seduced the other's fiancée. Ferrando is wounded. Guglielmo laughs — but bitterly.

Don Alfonso pushes things to the limit: he arranges a mock wedding between the swapped couples. In the middle of the ceremony, the original fiancés 'return'. They find the evidence of betrayal. Fury, tears, confessions. Then Don Alfonso reveals everything: it was a game. The original couples reunite — perhaps. Mozart does not tell us whether they are truly happy.