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.