An
opera by
Giuseppe Verdi, based on the factual event of the
assassination of King
Gustav III of Sweden at a
masked ball in 1792. This so horrified the
censors that Verdi was ordered to totally rewrite the scenario. It was eventually performed set in colonial
Boston. You could show a governor being murdered on stage -- but not a king.
The result is that we have an opera in which a number of main characters have alternative names. "Sam" and "Tom" in the version usually performed are Count Ribbing and Count Horn in the banned original. The king/governor is Riccardo: an Italianised form of an English name, but not Gustavo.
The core of the story, and the driving force of the assassination, is a love triangle. Well, that's in the opera. In real life Gustav III was operating a reformist policy domestically and allying himself with factions in France who... but this isn't going to sustain grand opera. We want love, betrayal, revenge, and misunderstanding. So. Riccardo the governor/king is in love with Amelia, wife of his best friend Renato. In real life Renato was the Swedish Count Ankerström, mixed up in the political intrigues between oligarchs and populists. Here, he's a trusted friend.
Amelia is tempted. She does not fall, but she is tempted. There is a fortune teller, Ulrica, and for a bit of sport the king/governor decides to visit her, in disguise, with a few of his mates tagging along. (He's not a bad person, not like the Duke in Rigoletto, who also does this disguised mixing with the peasants thing.) Ulrica foretells that he'll be killed by the next person to shake his hand. Riccardo regards this as absurd, and greets his newly-arrived dear friend Renato. Whose wife he wants to bang. But that doesn't ring any bells yet.
In Act 2 the sorely tempted virtuous Amelia, having consulted Ulrica about her inconvenient admiration for Riccardo, goes to pluck the magical herb she was advised to find. Riccardo has overheard, intercepts her, and presses his suit. In the meantime his beloved, faithful friend Renato has heard of a plot to kill him, and comes to warn him. Riccardo, possibly making an error of judgement at this point, asks the loyal Renato to escort the veiled Amelia home, having neglected to mention that Amelia is, should the veil lift, Renato's own wife.
They are accosted by the assassins (remember Sam and Tom?). Riccardo had conveniently suggested swapping cloaks with Renato, so if anything bad was going to happen, it would happen to... look, it's an opera, and they don't think quite the way we do. So anyway, the assassins bent on killing King Gustav/ Governor Richard lunge for this cloaked figure, when Amelia saves her husband by revealing herself.
At this point Renato works out what's been going on behind his back.
At this point Renato is in the company of (i) two assassins, Sam and Tom, who want to kill his friend and master; (ii) his wife, the object of lust of his friend and master. His eyes go all slitty. It doesn't say that as such in the libretto, but you know that's what happens. He joins the conspirators, and they force Amelia to draw lots for who's going to kill Gustavo/Riccardo at the forthcoming masked ball.
Un ballo in maschera had a libretto by the astonishingly fecund librettist Eugène Scribe (whose other creations include, just to name the most famous, La Sonnambula, Adriana Lecouvreur, La Juive, L'Elisir d'amore, Les Huguenots, Le Comte Ory, and Les Vêpres siciliennes). Verdi originally submitted it for performance at the San Carlo Theatre in Naples. The Neapolitan censors had apoplexy. Kill a king on stage? Are you nuts?? With the substitution of 1600s Boston for 1700s Stockholm it was first performed in Rome, on 17 February 1859.