The action begins when Ferrando and Guglielmo arrive at Don Alfonso’s bar with their girlfriends, Dorabella and Fiordiligi. The men immediately get into an argument about women. As the proprietor of the club, Don Alfonso has seen all kinds of relationships and is convinced that there is no such thing as a faithful woman, but the young men insist that Dorabella and Fiordiligi would never cheat on them. Provoked by their self-righteousness, Don Alfonso challenges the boys to a bet: he will prove in less than 24 hours that their girls are no different from all others. As part of the test, Ferrando and Guglielmo agree to do whatever Don Alfonso says.
In another room of the club, Fiordiligi and Dorabella compare snapshots of their boyfriends and gossip about marriage. The sisters begin to wonder what’s keeping Guglielmo and Ferrando, when Don Alfonso enters with the news that the boys have been unexpectedly summoned to join the war effort. The girls fall for the false story. The young men enact a tearful goodbye with the girls, whose despair seems proof enough of their fidelity. But Don Alfonso reminds Ferrando and Guglielmo that the game has just begun. A crowd of club goers joins the deception by giving the boys a patriotic sendoff. Left alone with Don Alfonso, Dorabella and Fiordiligi imagine their boyfriends sailing off to war and say a prayer for their safety.
Despina, an employee at the club, brings coffee to the V.I.P. room for the girls. Fiordiligi and Dorabella explode in a fit of grief over their boyfriends’ departure. Despina ridicules their sadness and urges them to enjoy new romantic opportunities just like their boyfriends will be doing away from home. Setting the next stage of his plan in motion, Don Alfonso bribes Despina to introduce two new guys (really Ferrando and Guglielmo disguised as Eurotrash) to the girls. Despina agrees without realizing the suitors’ true identity. When these foreign suitors hit on the girls, they are outraged. Now sure they have won the bet, the boys offer to reduce Don Alfonso’s debt to them. But the test is not over yet. Don Alfonso and Despina hatch a new plan: the suitors will pretend to commit suicide for love as a new tactic to weaken the girls’ resistance. After the boys apparently overdose in front of the girls, Despina, disguised as a doctor, cures the foreigners with her healing powers. The girls still refuse to yield but become suspiciously passionate in their rejection. The boys begin to wonder whether the anger is feigned or real.
Despina tries to convince Dorabella and Fiordiligi to live it up while the boys are off at war. Left alone, the girls fantasize about the foreigners and each admits to a crush, unknowingly picking the other’s boyfriend as the object of their affection. Don Alfonso rushes the girls to another room, where the boys await them with a serenade. The lovers pair off, the girls choosing the boy of their preference. When Dorabella is left alone with Guglielmo, she quickly succumbs; Fiordiligi refuses Ferrando. The boys reunite to report about the girls, and Guglielmo breaks the bad news to Ferrando. Later, Fiordiligi confesses to Dorabella and Despina that she has fallen in love with her foreign suitor, and Dorabella urges her sister to follow her passion. In a desperate last attempt to resist, Fiordiligi says she will join Guglielmo on the battlefield, but this plan falls apart when she and Ferrando meet again. Guglielmo witnesses Fiordiligi’s betrayal and is furious. When he talks of the violence he will do to her, Don Alfonso realizes the game is spinning dangerously out of control. Taking the reins, he proposes a double marriage for the couples, with Despina disguised as the notary. The couples comply and begin the ceremony until they are interrupted by the sound of the returning army. The girls hide their new Eurotrash husbands, who sneak off and reappear as Ferrando and Guglielmo back from war. Chaos ensues until Don Alfonso tries to reunite the original pairs—but the game is not so easily wrapped up. It has been a night of tears, passion, and betrayal. The lovers sing of finding “beautiful calm,” but the music’s turbulence points to a deep inner turmoil.