Scollatella rushes into a hall in the castle, dismayed because her clothing may have been ruined by the storm raging outside. She wishes to appear at her best for the ceremony at which the new king, Leandro, will choose a queen. Frightened at being alone, she calls on her doubles who leap from a mirror to keep her company. The sound of coronation bells is heard. Tartaglia, the Royal Chancellor, describes the ceremony as it takes place, heaping scorn on the courtiers who do homage to the young king. Two guards bring in Costanza who does not wish to participate in the forthcoming ceremony. After ascertaining that Costanza has never seen Leandro, Tartaglia describes him as a tyrant, and encourages Costanza to assassinate him, giving her a dagger for that purpose.

A group of forest animals precedes Leandro into the hall. He bids them farewell, and asks them to return to their forest where he was raised. As they leave, he sings of his loneliness in the world of men. Two statues agree to aid him; whenever anyone attempts to lie, they will laugh aloud. A trumpet fanfare signals the beginning of the ceremony at which Leandro will choose his queen. Scollatella and her doubles present themselves to the king, but soon become involved in an argument among themselves. Tartaglia calls for silence, and Leandro questions the ladies on the sincerity of their love for him. Their answers are greeted by peals of laughter from the statues. Costanza is summoned. She begins by berating Leandro, but ends by charming him so completely, and, in turn, falling in love with him. They exchange words of love; Leandro listens for laughter from the statues, but there is only silence. If Costanza were ever to be untruthful to him, he would not wish to know of it, so he destroys the statues.

Tartaglia now seizes the opportunity to accuse Costanza of plotting to kill the king, and has her carried off to prison. Leandro pleads for her freedom, but Tartaglia insists that the laws be upheld. Leandro renounces his throne in order to be able to forgive Costanza and leave the world of men. Cigolotti, a magician, appears and leads Leandro away to the forest. Coltellino (a professional assassin like his father, although a timid one) is ordered by Tartaglia to follow the king and to kill him. Six Alchemists, carrying fantastic but useless inventions, enter in search of Leandro. They wish to present a performance for him, but are told that he has returned to the forest.

Leandro, led by Cigolotti, wanders through a dark forest. Tartaglia, concerned that Leandro may change his mind and return to the city to claim his throne, follows at a distance. Checco, a minstrel, searching for a parrot, hides behind a tree as he hears Coltellino approaching. As Leandro enters the clearing, Tartaglia throws a dagger at him, but misses. The alchemists, disguised as animals, enter the clearing. They are afraid that they will be shot by hunters, but, on the other hand, they fear that they will be attacked by wild animals if they remove their animal costumes. They run off in a quandary.

Cigolotti awakens the sleeping Checco to warn him of Leandro’s danger, and to give him an eye of crystal in which he will be able to see the king. He also gives Checco a magic formula which should be carried to the king by wind spirits when he needs help. Coltellino rushes in, pursued by the Alchemists. As he and Checco scramble into the trees, a beautiful stag enters the clearing. As it makes its escape, it is wounded by Tartaglia. Seeing Checco, Tartaglia tells him that Leandro is in danger, and forces him to reveal the secret of the crystal eye. As Checco raises the eye, Leandro is seen entering a distant clearing. He pleads for freedom from human cares. Tartaglia listens carefully as Checco whispers the magic formula to the wind spirits. When they raise the eye once more, they see Leandro repeating the magic formula given to him by the wind spirits. He falls to the ground as if dead, then raises up again in the body of the stag which Tartaglia had fatally wounded. Tartaglia rushes to the clearing, and recites the formula over Leandra’s corpse. He falls to the ground, then rises again in Leandro’s body. With a wild laugh, he draws his sword, and severs the head from the Tartaglia-corpse. Now in the body of Leandro, Tartaglia praises Might, Violence and Evil. Checco accuses him of having stolen the body of the king, but Tartaglia has him put in chains. When Tartaglia orders his hunters to capture the stag, the forest closes itself in defense of the king, forcing Tartaglia to retreat to the castle.

Evil elements have gained control of the city, but the townspeople have not lost hope. Whispering voices repeat a legend which warns that when a stag walks through the streets, murderers will die. Costanza and the stag meet in the city square. She feels a strange affinity to the animal, but as she approaches, it flees. Tartaglia, a victim of persecution mania, sees Costanza and demands to know why she has hidden herself from him. As he tries to embrace her, she runs off. The townspeople fill the square to watch for the stag which has been seen near the city. Suddenly the stag appears and there is a moment of hushed tension as Costanza also returns. Tartaglia aims at the stag and shoots, as Coltellino, his arm guided by Cigolotti, shoots at Tartaglia. As Tartaglia sinks to the ground, Coltellino runs off in horror, believing he has finally killed the king. Leandro recites the magic formula over the corpse, falls to the ground, and regains his own body. He and Costanza again exchange promises of eternal love.