Young friends Ferrando and Guglielmo are in love with two sisters, Dorabella and Fiordiligi respectively. They are deeply offended when their older friend, the philosopher Don Alfonso, claims that all women are fickle and that their young fiancées are incapable of fidelity. They demand he prove his point or submit to a duel. A wager is placed and the boys agree to do everything Don Alfonso asks for twenty four hours. He has a plan to pretend the boys have gone to war but they will return in disguise and attempt to seduce the other’s lover.

At the house of their fiancées, we hear the sisters muse (somewhat competitively) on the qualities of their betrothed. Don Alfonso bursts in with bad news. Ferrando and Guglielmo have been called up to the front line and must leave immediately for battle. The boys arrive and exchange tearful goodbyes. Don Alfonso “comforts” the girls and leaves them in distress, extremely pleased with himself.

Despina, the girls’ maid and confidante, arrives. Seeing their sorry state, she tries to cheer them up by suggesting that to put their faith in the fidelity of soldiers is foolish and that this is an opportunity for them to enjoy themselves. The girls are incensed and depart.

Don Alfonso returns, this time to convince Despina to help him introduce two young men to the girls to distract them. He brings out Guglielmo and Ferrando, now disguised with outlandish outfits and moustaches. Despina laughs at them, but gamely helps out.

The girls are horrified. Fiordiligi tells the new arrivals, in no uncertain terms, that she is like a rock and will not have it.

This pleases the boys no end, and they crow at Don Alfonso. He and Despina hatch a plan and sneak the boys back in, this time claiming to be dying of love, having taken arsenic. The girls call for Despina, and she instructs them to cradle the boys’ heads while she fetches a doctor. A strangely familiar doctor appears, spouting nonsense about a miraculous magnet therapy cure which magically brings them back to life. Despite pleading from the “doctor”, Don Alfonso and the moustachioed strangers, the girls refuse to kiss them, for now at least…

Despina tries again to convince the girls to relax and that love should not be so serious. This time the girls are a little tempted and decide which of the suitors they’d like to flirt with.

The boys return and a double date begins - a walk in the gardens. Bolder than her sister, Dorabella ends up in a love duet with Guglielmo, and they exchange lockets. Ferrando has less success with Fiordiligi. When she commands him to leave, she is confused by her feelings but vows to remain faithful.

Ferrando praises Guglielmo for Fiordiligi’s fidelity, before Guglielmo gives Ferrando the bad news about how easily his medallion was given away.

Despina and Dorabella celebrate her becoming more of a “woman of the world”, which distresses Fiordiligi. She dresses herself in one of Ferrando’s uniforms ready to join Guglielmo at the front. Ferrando interrupts her and finally successfully wears her down. This means that Alfonso has won the bet, and all three men agree “così fan tutte”- all women are like that!

The girls agree to marry the new suitors. After some time-wasting from a very familiar notary, military music is heard in the distance - the army (with Ferrando and Guglielmo) has returned! The new suitors leave, the notary hides, and Don Alfonso tells the girls not to worry.

Enter the original lovers and, piece by piece, the deception is revealed. Don Alfonso is forgiven for masterminding it all, and they all live happily ever after… but with which partners?