As the play opens two strangers whose storm-frighted horses have bolted, stumble on Segismund’s prison. One of them confesses in a voice all too gentle for her masculine attire that she has come from Muscovy on a matter of vengeance and Segismund, for the moment unguarded, confesses that he too, thinks often on revenge. Clotaldo’s appearance is about to result in death for the newcomers when the general recognizes the stranger’s sword as one he had left years before in Muscovy as pledge for favor owed. The stranger identifies herself as Rosaura, daughter of Clotaldo’s quondam benefactor, and is proffered safe conduct to Warsaw.

Meanwhile the King has Segismund brought to court while in a drugged sleep, to wake to all the appearances of royal splendor. His tragic story is related to him, he meets his cousins, Astolfo and Estrella, and falls promptly in love with the latter. When, however, his father, the King, appears, his desire for revenge on an unnatural father is too strong and he would have attacked the King had not the guards prevented. For this action he is returned in a drugged sleep to his prison and the King prepares to carry out his plans to marry his nephew, Duke Astolfo of Muscovy, to his niece, Estrella, and turn over his kingdom to them.

Meanwhile, back in the prison, Segismund is convinced by Clotaldo that the entire day’s happenings are but a dream. Clotaldo nevertheless chides him for his unprincelike lack of self-control so effectively that when later in the day he is rescued by revolting Polish troops directed to his prison by Rosaura, he treats the vanquished King with great nobility and returns to him his forfeit crown. When he discovers that Astolfo has broken his engagement to Rosaura in hopes of gaining the Polish crown through marriage to Estrella, he dissolves the new bond, returning Astolfo to Rosaura and claims Estrella for himself.