Act I.

Ferrando and Guglielmo, two soldiers, believe their fiancées, sisters named Dorabella and Fiordiligi, are incapable of infidelity. Don Alfonso is more jaded: “Are they goddesses or women?” he asks. The lovers are offended and want to fight. Don Alfonso suggests a wager instead. He offers them a hundred sequins• if he can’t prove that very day that these women are like all others—fickle. His only stipulation is that the men do everything he asks. They agree.

Fiordiligi and Dorabella admire their fiancés’ portraits and dream of getting married. Don Alfonso tells them that their soldiers are off to battle. As the couples part, the men secretly enjoy their sweethearts’ sorrow. The sisters and Don Alfonso pray for a safe journey.

Despina, the maid, prepares hot chocolate. The desperate Fiordiligi seeks a dagger; Dorabella threatens to die from rage and sorrow. Despina advises the sisters to enjoy themselves as she is sure their fiancés are doing. “Do you expect fidelity from soldiers?” she asks.

Don Alfonso, afraid that Despina will see through his plot, bribes her to encourage her mistresses to receive two visitors—Ferrando and Guglielmo in disguise. The sisters are horrified to have men in their house until Don Alfonso assures them that these are his friends. The disguised men declare undying love: Dorabella is confused, but Fiordiligi protests that she is impervious to any attempts to sway her constancy. Guglielmo pleads for attention.

After the sisters leave, Ferrando and Guglielmo, in high spirits, offer to let Don Alfonso off lightly. He reminds them that the day isn’t over. The men have built an appetite, but, as Ferrando says, just being around their sweethearts is nourishing.

The women, upset, become more so when the disguised lovers poison themselves. A doctor—Despina in disguise—uses magnets to cure the supposed suicides. The recovering suitors ask for a kiss and are rejected.

Act II

Despina tells the sisters to stop being silly and to enjoy themselves. To protect their reputations, she’ll let it be known that the suitors are visiting her. Any fifteen-year-old girl should know how to handle such things!

Dorabella is swayed and decides to take the “dark one”—Guglielmo! She tells Fiordiligi that their hearts won’t change: Having fun while their fiancés are away is better than moping.

Don Alfonso and Despina prod the couples along. Each sister goes for a stroll with the other’s fiancé. Guglielmo succeeds in winning Dorabella’s affections—she allows him to give her a heart pendant in exchange for her locket with Ferrando’s portrait. Ferrando doesn’t fare as well with Fiordiligi, who, while attracted to him, is torn by guilt.

Ferrando rejoices with Guglielmo that Fiordiligi has remained constant—until he learns that Dorabella has not. Guglielmo is somewhat disillusioned, but Ferrando is crushed by the betrayal.

Fiordiligi is in torment because she is not indifferent to her new suitor. Dorabella insists that she should give in to her feelings. Instead, Fiordiligi decides to visit Guglielmo on the battlefield. She sends Despina to fetch uniforms so the sisters can disguise themselves as soldiers.

Dressed in Ferrando’s uniform, Fiordiligi is surprised by Ferrando, who demands she love him or to run him through with a sword. She yields.

It is now Guglielmo’s turn to feel betrayed. The men wonder how they should punish their unfaithful fiancés. “Marry them,” Don Alfonso counsels. If these women can be fickle, imagine what others might be like. Everyone blames women, but he doesn’t, because he isn’t deceived about their nature: Women just are the way they are. “Così fan tutte.”

Despina reports that the sisters have decided to get married. She disguises herself again—as the notary. As soon as the wedding contracts are signed Ferrando and Guglielmo return without their disguises. They pretend outrage and leave to pursue the supposed suitors, returning half disguised. All three women are stunned at having been fooled. The sisters are embarrassed and sorry. Don Alfonso tells them he deceived them only to undeceive their fiancés. The four lovers quickly decide to look on the bright side of life.