The action takes place at nightfall in a mountainous region of Poland. A lady named Rosaura enters, dressed as a man, followed by her servant, Fife, similarly attired. Their horses have run away, and they are cursing their bad luck at being in a strange land, bereft of food, shelter, or transport. They see a small tower with an open door and decide to seek shelter there. As they approach, they hear the sound of chains rattling.

Entering the tower, Rosaura and Fife see Segismund, in chains and dressed in animal skins. They think he looks wild and dangerous, but Rosaura also sees a certain grandeur in him. Segismund is lamenting his fate, saying that his only crime was that of being born. This is something he has in common with all living creatures, but, he complains, he is punished with unique harshness. As they listen to Segismund’s words, Fife remains wary and comments sardonically on his complaints, but Rosaura is filled with sympathy. She begins to speak to Segismund and to question him about his situation, but before he is able to answer, Clotaldo, who has been charged by the king with guarding Segismund, enters the tower with his soldiers. Rosaura flings her sword towards Segismund, crying out to him to take it so that he can defend himself.

When Clotaldo sees the sword, he orders the soldiers to bring him the intruder who gave it to Segismund. The soldiers find Rosaura and Fife, and Clotaldo questions them. Rosaura says that they come from Muscovy and are on a journey to the Polish capital, Warsaw, though she will not reveal their purpose. Clotaldo asks where she obtained the sword, and Rosaura says that she got it from her father, who received it from a Polish man as a pledge of his return to Muscovy. Clotaldo recognizes the sword as his own, which he gave many years before to a man who saved his life when he was dying on the battlefield. He concludes that Rosaura (who is dressed as a man) must be the son of his benefactor and determines to take her to the capital and present her to the king. He will beg a royal pardon for Rosaura’s unwitting offense in entering the tower and do all he can to help her.

In the royal palace, Astolfo, duke of Muscovy, discusses the succession to the Polish crown with Princess Estrella, his cousin. Each of them has a faction supporting their claim to the succession, and the king, Basilio, has invited them to court today to resolve the matter and prevent violence.

King Basilio enters and, in a long speech, tells Astolfo and Estrella about the birth of his son, Segismund. His wife, Queen Clorilena, died in childbirth, and Basilius foresaw through his studies of astrology and the occult that his son would be a cruel and evil man who would tear the country apart and kill Basilius in the process. Basilius therefore told everyone that his son had died along with Clorilena and secretly commanded a tower to be built in the remote mountains, where he had the young prince imprisoned. However, he is concerned that he may have wronged his son by placing too much faith in his own ability to predict the future. Therefore, he has decided that Segismund is to be brought to the palace and acknowledged as the prince. If he shows himself to be kindly and humane, Basilio will apologize and pass the crown on to his son. However, if he turns out to be as cruel and savage as Basilio believed he would, then he will be returned to his dungeon. Astolfo and Estrella both approve the king’s plan, as do his courtiers. It is therefore arranged that the prince will be given a powerful sleeping draught and brought to the palace.