Mozart's comic opera Cosí fan tutte (That's the way woman are)

Two brothers, Guglielmo and Ferrando, are absolutely certain of winning a bet proposed by the cynical Don Alfonso that, unlike all other women, their fianceés – two sisters, Fiordiligi and Dorabella – would always be true and faithful. They agree to pretend to be called away to war, planning to return in disguise to try and seduce each others' loves. The two couples bid each other a tearful farewell. Despina, the sisters’ maid suggests they amuse themselves in their lovers’ absence, a notion that shocks them. Don Alfonso secretly asks Despina to play along with his scheme. She invites the disguised brothers into the house. Fiordiligi and Dorabella are disturbed at this, especially when the two “strangers” begin wooing them. Don Alfonso pretends they are old friends, whom the sisters should welcome, but Fiordiligi staunchly maintains her fidelity to the absent Guglielmo. The two “strangers” reappear, apparently take poison and collapse. Despina and Don Alfonso leave to get help. A doctor (Despina in disguise) arrives. The recovering suitors proclaim their love to the sisters, who are not to be swayed.
Despina convinces Fiordiligi and Dorabella that a bit of flirtation would do no harm. Dorabella decides on Guglielmo while Fiordiligi chooses Ferrando, an unwitting crossover of lovers. Fiordiligi goes off for a stroll with Ferrando while Guglielmo woos Dorabella. To his amazement, she surrenders to his advances. In contrast, Fiordiligi continues to resist Ferrando. When the two brothers compare notes, Ferrando is tormented to discover that Dorabella has yielded to Guglielmo. Secretly observed by Don Alfonso and Guglielmo, Ferrando renews his efforts to win Fiordiligi. To Guglielmo’s distress, he succeeds. Don Alfonso has won his bet. As consolation, he suggests ‘così fan tutte’ (‘all women are like that’). The double wedding is underway when a military chorus off-stage heralds that the “soldiers” are “returning” which prompts the suitors and the notary (Despina in disguise!) to hide. Ferrando and Guglielmo then appear in uniform, seemingly taken aback by their uncertain reception. On discovering what has happened in their “absence”, they swear vengeance, but ultimately reveal the deception. In an operatic happy ending, the two pairs of lovers are seemingly reconciled.