A wedding chorus proclaims the upcoming wedding of Isoletta, daughter of Montolino, to Count Arturo of Ravenstal. Isoletta fears her Arturo's changed attitude toward her. She tells Baron Valdeburgo that she believes Arturo has fallen in love with a mysterious woman living as a hermit in a hut by the lake. In the distance a crowd is heard following "la straniera" on the lake shore, cursing her as a witch. Count Montolino shares his daughter Isoletta's concerns, but is reassured by his friend Osburgo who promises to bring Arturo to his senses.
Arturo is at the hut of "la straniera" Alaide. He desires to know the identity of this mysterious woman. Inside the hut he sees a portrait of her dressed in royal robes, wearing jewels. Alaide enters and chastises Arturo for entering her hut. She tells him she will reveal nothing about her past and begs him never to return. She does, however, admit to an attraction to Arturo. She sends him away, warning that his persistence will be their ruin.
During a hunting expedition, Osburgo and Valdeburgo encounter Arturo. Osburgo begs him to return for his wedding to Isoletta. Arturo refuses, asking Valdeburgo to meet his true love. Arturo promises he will never see her again if Valdeburgo judges her unworthy. Valdeburgo, upon seeing Alaide, hails her and almost calls out her real name, but Alaide stops him. Valdeburgo tells Arturo that-for reasons he cannot reveal-Arturo must renounce any intentions toward Alaide. She cannot ever marry Arturo. Thinking Valdeburgo is his rival for Alaide's affections, Arturo attacks him with his sword. Alaide intervenes and begs Arturo to leave. She agrees to see him again-if for the last time.
Arturo, still mistakenly crazy with jealousy directed at Valdeburgo, is further infuriated when Osburgo tells him that he has overheard Valdeburgo and Alaide planning to flee together. Arturo duels with Valdeburgo who is wounded by Arturo's sword and falls in the lake. Alaide then appears and Arturo curses her. Alaide reveals that Valdeburgo is actually her brother. Arturo jumps in the lake in an effort to save Valdeburgo. A crowd attracted by the shouting finds Alaide standing with Arturo's bloody sword. They accuse her of murdering Valdeburgo and drag her off as a prisoner.
Alaide is brought to trial. Osburgo testifies against her. The presiding Priore (Prior) asks her name, and she responds only "la straniera." The Prior feels that he has heard her voice before. Arturo rushes in and proclaims her innocence and confesses his own guilt. However, Valdeburgo suddenly appears as well to announce that Arturo is innocent. The Prior again demands Alaide's name, which she refuses to reveal. But she does agree to lift her veil for the Prior. He gasps upon seeing her face and sends her off with Valdeburgo. Arturo is left alone, while the Prior chastises Osburgo for his false testimony against Alaide.
Arturo comes to beg Alaide's forgiveness and confess his love. But he encounters Valdeburgo outside her hut. Valdeburgo again pleads with Arturo to desist in his attentions toward Alaide. Arturo reluctantly agrees to return to marry Isoletta, but asks that Alaide attend his wedding so he can see her one last time. Valdeburgo agrees.
Isoletta, truly unhappy and understandably feeling ignored and unloved, prepares for her wedding. The wedding party appears. Valdeburgo tells Arturo that Alaide is present, but hidden. Arturo behaves badly toward Isoletta who threatens to stop the wedding. Alaide, Valdeburgo, Arturo and Isoletta all weep over their respective torments. Isoletta finally renounces Arturo. Alaide suddenly reveals herself and begs Isoletta to continue with the wedding, and starts to rush out of the church. Arturo abandons Isoletta at the altar and begs Alaide to run off with him. The Prior then announces to all that Alaide is in fact Queen Agnes. What's more, the Prior has just learned that the Queen's rival for the throne has died and she must now return to Paris. Arturo, rendered mad by this news, throws himself on his sword. Isoletta falls on his dead body. La Straniera/Alaide/Agnes must now return to fulfill her duty as Queen, abandoning all hope of personal happiness.