King for a Day
Original Title: Un giorno di regno
Barone di Kelbar's castle near Brest, in France.
Cavalier Belfiore, an officer in the service of the new King Stanislaus I of Poland, has been in France posing as the King. This has allowed the real Stanislaus to travel to Poland in order to stake his claim to the throne. Belfiore, now a guest of Baron Kelbar, has been invited to take part in two imminent marriages: the first is between Kelbar's daughter Giulietta and the Grand Treasurer, the wealthy and greedy Gasparo Antonio La Rocca. The second is between Kelbar's niece, the Marchioness of Poggio, and Count Ivrea. Neither marriage is exactly made in heaven. The Marchioness has agreed to hers out of spite, because she has stopped hearing from her fiancé—none other than Cavalier Belfiore. He cannot reveal his true identity and assuage her fears because of his pledge to the King. The other pairing is equally unfortunate: Edoardo, a young officer and La Rocca's nephew, is in love with Giulietta, but realizes he will not be able to marry her because of his uncle. He begs "the King" to allow him to enlist in the Polish army and follow him to Poland so that he may remove himself from his torment. Belfiore, hoping to turn this to his advantage, agrees.
Belfiore also tries to help Edoardo by persuading the vain and greedy La Rocca not to marry Giulietta, bribing him with a more financially advantageous arrangement together with a post in the Polish government. When La Rocca consequently refuses the marriage contract to Giulietta, Baron Kelbar challenges him to a duel. The Marchioness proposes that Kelbar instead seek real revenge and marry Giulietta off to the younger Edoardo in order to humiliate La Rocca. Kelbar and La Rocca are about to fight when "the King" forbids any altercation and declares it his right to make all decisions.
In Act Two, Belfiore (still disguised as King Stanislaus) orders La Rocca to give his nephew Edoardo a castle and allowance in order to end the latter's poverty. La Rocca refuses and prepares to duel with the Baron.
The Marchioness is enraged by Belfiore's refusal to acknowledge his true identity. She and Belfiore tease each other mercilessly -- they exchange jibes about the one-time relationship between Count Belfiore and the Marchioness as if he is not really there, each keeping up the pretense. The Marchioness tries to provoke Belfiore into abandoning his disguise by pretending to love Count Ivrea. The fake Stanislaus says that Cavalier Belfiore will come to claim her hand before it is too late. Desperate to be given royal permission to drop the pretense, Belfiore declares he must leave at once for Poland and that Edoardo must come with him. This causes major consternation, especially to Edoardo and Giulietta.
But at the last minute, a letter arrives from the real King Stanislaus, who has been crowned in Warsaw, relieving Belfiore from his duties as Official Imposter. Before sharing the letter's content, Belfiore orders the marriage of Giulietta and Edoardo to be celebrated. He then reveals his true identity and declares himself faithful to the Marchioness, while (almost) everyone rejoices.