Sigismondo, dramma per musica in two acts to a libretto by Giuseppe Foppa, was first performed at Teatro La Fenice, Venice, on 26th December 1814. The singers in the first performances were Maria Marcolini (Sigismondo), Luciano Bianchi (Ulderico), Elisabetta Manfredini-Guarmani (Aldimira), Claudio Bonoldi (Ladislao), Marianna Rossi (Anagilda), Luciano Bianchi (Zenovito), Domenico Bartoli (Radoski).
At Gesna, capital of Poland, in King Sigismondo’s apartments, Ladislao’s sister Anagilda and Radoski, the King’s adviser, sadly discuss their sovereign’s fate, subject as he is to frequent attacks of madness. Ladislao pretends to be worried about Sigismondo and about the throne of Poland, but he is really only interested in having Anagilda proclaimed Queen.
Loud shrieks proclaim the arrival of the King, who comes out of his rooms raving, as though he were being pursued by a ghost. When anyone tries to ask him what is wrong, he does not answer. Anagilda leaves with Radoski after expressing once more her esteem for her Sovereign. Left alone with Ladislao, Sigismondo tells him that it is the ghost of Aldimira that is persecuting him. Now he is full of remorse: what if he had been mistaken? What if both of them had been victims of a plot? Ladislao trembles at the idea of being found out. But Sigismondo is thinking over a plan that will put an end to all his suffering: hearing that King Ulderico of Bohemia is leading his army towards Poland in order to avenge the death of his daughter Aldimira, he has decided to send Ladislao to organize the defences whilst he himself will pretend to go on a hunting expedition into the woods that separate the two countries; there he will engage the enemy and find the death he longs for, whilst the Polish army will gain time to prepare the counter-attack.
The scene changes to the countryside on the edge of the forest, with a rustic cottage. Aldimira blesses the quiet spot that shelters her, without, however, being able to bring her peace: one man only could do that for her, and he is the very one who had condemned her to death. Zenovito, a Polish nobleman who lives alone in the place, comes out of the cottage. From their conversation we learn the story of Ladislao’s insane desire for Aldimira, and how Zenovito came to the assistance of the poor woman, dragged by a band of ruffians to the edge of the precipice; now he looks after the Queen’s safety, passing her off as his daughter Egelinda.
Noises off cause Aldimira to conceal herself in the cottage, from which she can spy out: some hunters rush past. Zenovito learns that they belong to the royal party, and that Sigismondo himself is on his way. At last he comes on: while Aldimira and Zenovito watch from their hiding-place, the King has another of his visions. Ladislao’s arrival brings him back to earth: Ulderico is advancing rapidly, and will soon be at the palace if the defences are not re-inforced. Sigismondo therefore sends Ladislao into the house to gain information about the surroundings from the inhabitants. Ladislao also goes in, but quickly comes out again, deeply disturbed. Aldimira also comes out and kneels at the feet of the King, while Ladislao runs away, convinced that he has seen a ghost. The lady introduces herself as Egelinda, a fugitive from court together with her father Zenovito because of a cruel plot. Sigismondo can hardly believe his eyes. The pair remain for some time each caught up in his own thoughts. The King goes off into the woods talking to Zenovito, while Aldimira goes back indoors.
Just then Ladislao comes back, seeking in vain for Sigismondo. Instead he meets Zenovito, who announces the King’s extraordinary new decision: taking advantage of the incredible resemblance between Egelinda and the dead Aldimira, the one will take the place of the other, and when she has been conducted to court she will assume the clothing and the rôle of the queen, which will placate the wrath of Ulderico, who will have to believe that he was wrongly informed as to his daughter’s fate. Ladislao is appalled and runs off to find the King and stop him from carrying out this unexpected scheme; but he soon comes back: instead of listening to him, Sigismondo has commissioned him, ironically, to take charge of conducting Egelinda to the palace. However, Zenovito tells him that the lady refuses to leave. Finding himself alone with her, Ladislao is quelled by the imperious attitude of Egelinda, who orders him to provide securities for her safety.
When Anagilda and Radoski come on, the whole court is assembled in the humble dwelling: Sigismondo cannot bear Egelinda’s searching glance; she fears for her own safety and Ladislao is increasingly afraid that he will be unmasked. The King has barely the time to personally entreat the lady to follow him to the palace when soldiers burst in, calling Sigismondo to engage battle with the Bohemian troops who have already invaded the surrounding wood.
While Ulderico is about to reach Gesna, in the royal palace Sigismondo presents Egelinda dressed up as Aldimira. There is general rejoicing for the rediscovered Queen, not shared however by Ladislao, who sees his dreams of his sister Anagilda’s ascending the throne evaporate.
Left alone together, Aldimira and Sigismondo can scarcely restrain their feelings: she would like to know the truth behind her unmotivated condemnation to death; he is uneasy when he feels his old love renewing itself for an unknown woman, however he offers her his ring and his throne. Agitated, they hurry away separately.
Meanwhile Radoski has cleared up the mystery of the past: he has got hold of a document that proves Ladislao’s guilt, but is not yet ready to reveal it. On the contrary, meeting Anagilda, he comforts her by predicting that , once the military emergency is resolved, Sigismondo will certainly send Zenovito’s daughter back to the woods and the way to the throne will once more be open to her. Ladislao asks Radoski to confirm that Aldimira had really and truly been sent to her death; then he is overcome with dread and prays for peace for his conscience.
Radoski secretly gives Aldimira the letter that Ladislao had once written to her, trying to seduce her, while Sigismondo tells her that Ulderico is about to arrive and reminds her of the part she will have to play; she reassures him.
In a valley outside the city, the King of Bohemia is awaiting the Polish Prime Minister: if it is true that his daughter is still alive, he will once more be a friend and defender of Poland. But Ladisalo reveals to Ulderico the change of one queen for another. When Sigismondo arrives, after a moment of hesitation caused by the incredible resemblance of the new Queen to his daughter, Ulderico lets all his rage burst out, addressing her as Egelinda: tempers fly and the two armies attack one another.
Radoski is made prisoner by Ulderico; Ladislao pleads for Radoski’s life, and Radowski is embittered by the idea of owing his life to such a rogue. Ulderico promises Ladislao that if he manages to discover Egelinda, she is to be completely in his power.
The Polish army is now on the verge of breaking up; Sigismondo attempts one last attack, but he is disarmed by Ulderico. Aldemira now runs on, screaming, hotly pursued by Ladislao, who trips on the slope and rolls down: stopped by the Bohemian guards, in his confusion he reveals his crimes. Sigismondo wants to jump on him, but is held back; he is calmed by Aldimira’s assurances that her old love for him is unchanged. To remove all doubts about her conduct from the minds of her father and her husband, she shows them the letter that Radoski had given her: the authentic Aldimira is the only person who could have come by it. Sigismondo, still doubting his good fortune, is reunited with his true wife. Ladislao’s life is spared but he is sent to prison. Everyone rejoices: each person in turn expresses his feelings on the occasion of the return of peace.