Two young officers, Ferrando and Guiglielmo, boast the unending fidelity of their fiancees in a tavern. An old philosopher, Don Alfonso, bets them that he can get the girls to betray them for other men after just a day. Confident in the virtue of their beloveds, the pair agree. Their fiancees are two noble ladies called Fiordiligi and Dorabella, sisters and apparently completely infatuated with their lovers. Their happiness is crushed when they are told that the soldiers are being called away to war, perhaps to never return away. Their world-wise maid Despina advises them that men - especially soldiers - are hardly faithful and that women should pursue their pleasure just as freely, much to their horror.
Of course, all this is just a ruse. The soldiers disguise themselves as Albanians and return to woo the sisters. Alfonso, worried that the sharp-tongued Despina might recognize them anyway, decides to bribe her over to their side. He needn't bother, because Despina doesn't recognize them and even if she did, she would likely help for her own amusement. The sisters arrive to scold Despina, only for each of them to be wooed by the disguised fiancé of the other. Alfonso returns and quickly establishes that these two are his best friends.
Fiordiligi will have none of it, though, and sends them both away, which doesn't help much. Alfonso despairs that he may have found the rare breed of a constant woman, but Despina, tempted by the promise of cash, devises a plan; the 'Albanians' burst into the room where the sisters are mourning and threaten to kill themselves with arsenic. Once they do, a doctor (Despina in disguise) revives them with a magnet and they try once again to play on the sisters' sympathies.
In act two, Despina urges the sisters to do as they please. Dorabella is tempted and the sisters eventually agree that they can flirt, as long as they break it off the moment their fiances return. Unbeknownst to them, each chooses the fiance of the other as the man she'll allow to woo her. Eventually, both of them succumb to these overtures - Dorabella eagerly, Fiordiligi much later and only after a long and difficult aria about how she hopes her fiancé will forgive this 'folly of the heart' (and when Ferrando foils her plan to disguise herself as a man to follow Guiglielmo to war) - much to the shame and anger of their fiances. Don Alfonso, however, points out that women are naturally flirtatious and shouldn't be blamed; it's the men's fault for not being too appreciative and attentive to them.
The finale involves a double wedding with Despina disguised as the notary, with the soldiers 'returning' just as the Albanians rush off to hide. As this is an opera buffa, the girls realize they were tricked, but Alfonso shrugs off the guilt: "I deceived you, but undeceived your beloveds; now they will love you for who you are, not how they dreamed you up!" Even Despina feels some shame at her free-wheeling philosophy and the general happy ending comes along.
Contains examples of:
- All Women Are Lustful: Taken to the extent of I'm A Woman, I Can't Help It. According to Despina, "You can live without love, but not without lovers."
- Despina claims that All Men Are Perverts (especially soldiers) to justify the cheating.
- Ambiguous Ending: Mozart and Da Ponte never specify which man ends up with which sister at the end of the opera, which has left directors free to choose any pairing they want, or to leave it unresolved.
- Author Avatar: Some studies suppose that Despina's lines allowed da Ponte to speak freely, with her being a servant.
- Bed Trick: This is one of the main premises, except brought down to PG proportions.
- Bad Girl Song / Sidekick Song: Both Despina's arias about her philosophy as a Good Bad Girl (by the standards of the time).
- Broken Bird: Arguably, Fiordiligi is the one who genuinely believes she loves her fiancé... and then she actually does fall in love with her new "suitor".
- Coupled Couples
- Divergent Character Evolution: Fiordiligi and Dorabella have basically the same personality in Act 1, and frequently sing the same words. In Act 2, as they start to fall for their new suitors, Dorabella begins to get more lighthearted music while Fiordiligi's becomes more passionate.
- The Hedonist: Despina could be considered one if one takes the time when the story takes place into account.
- "I Am" Song: Come scoglio (Like a rock), Fiordiligi's big aria about the sisters' faithfulness. Subverted later on.
- Innocent Soprano: The gentle and naive Fiordiligi is a high soprano, who struggles with her conscience more than her lower-voiced sister.
- Ironic Echo: "What woman can resist a Gugliemo" becomes "What woman can resist a Ferrando?" later.
- Jealous Romantic Witness: Guglielmo and Ferrando decide to test their respective fiancees' fidelity and try to seduce them in disguise. As a result, each of the girls falls in love with the other one's fiance. At first Guglielmo's fiancee Fiordiligi is determined to stay faithful, but then Guglielmo is forced to watch her duet with Fernando when her resolve finally breaks, and he is absolutely heartbroken and nearly runs out to confront the couple at once (Don Alfonso, who instigated the whole thing, has to hold him back).
- Large Ham: How Despina usually gets away with her interpretations of doctors and notaries, along with nonsensical Latin phrases and odd accents.
- Leaning on the Fourth Wall: Guglielmo's tirade against faithless women.
- Moral Myopia: In the 1983 Dresden production, Guglielmo and Ferrando are ready to fight for the honor of their fiancées... while amusing themselves with a crowd of girls.
- Notary Nonsense: Despina, the maid who encouraged the two female leads to embrace the same freedom men have and leave their old lovers as presumed dead and get with these new foreign suitors (actually the old lovers in disguise, seducing each other's girlfriends) dresses up as a notary to officiate once the women fall for the "new" men.
- Paper-Thin Disguise: Despina's diguises as a doctor and a notary.
- Sarcasm Mode: Many modern adaptations have the two heroes use this at the end, when returning to their faithful fiancées and their good friend Don Alfonso.
- Seduction as One-Upmanship: The plot involves two lovey-dovey couples, Gugliemo and Fiordiligi and Ferrando and Dorabella, the two men leaving their beloveds and returning in disguise attempting to seduce the other's betrothed at the urging of Don Alfonso. Gugliemo very quickly succeeds in sweeping Dorabella off her feet, while Ferrando encounters no success. When a triumphant Ferrando arrives expecting that his own fiancée was just as faithful to him, Gugliemo corrects him rather tactlessly, leading Ferrando to go after Fiordiligi again, succeeding this time (and proving Don Alfonso right: the opera's title means "Thus do women all act").
- Setting Update: A 1995 film version turns Don Alfonso into a sleazy diner owner, Despina into the waitress, and the Albanians into hippies (and the poison they drink? The mustard and ketchup bottles).
- Ship Sinking / Ship Tease: When Don Alfonso comes to Despina and tells her he needs a favor, her first response is: "An old guy like you and a young girl like me can do nothing together."
- Smug Snake: Gugliemo briefly turns into one when it seems only he's succeeded in seducing his friend's fiancée. Gets thrown back in his face later.
- Sweet Polly Oliver: Despina, whenever crossdressing as a man. Fiordiligi plans to do this to join her fiancé in the war as a soldier, but Ferrando manages to woo her before she can make good on that.
- Take That!: Despina, disguised as a (male) doctor, cured the heroes' faked arsenic poisoning by waving a magnet over their bodies. This was a dig at Franz Anton Mesmer's claims of "animal magnetism," which were in vogue at the time the opera was written.
- Title Drop: In the Aesop.
- Trickster Mentor: Don Alfonso.
- Wig, Dress, Accent: How the fiances disguise themselves.