Events unfold in Pisa at the end of the XV century. Guido Colonna, the head of the leaning garrison of Pisa, is discussing the situation with his lieutenants Borso and Torello: Pisa is surrounded by enemies - the troops of the Florentines, and the troops sent by Venice to help the Pisans could not get through to them. Famine is about to begin in the city. The soldiers did not have any gunpowder or bullets. Guido sent his father Marco to negotiate with Princivally, a mercenary commander of the Florentine army. There are various rumors about Princall: it is portrayed as cruel and insidious, then dangerous, but honest and noble. Marco is returning. He says that Princivalle received him as an honored guest. Marco tells with enthusiasm how he talked with Princivalle about the dialogues of Plato, how he met the famous scientist Ficino in the camping tent of the Florentine commander, how they together managed to find the torso of a goddess buried in sand in an olive grove ...

Guido interrupts the story of his father, trying to find out how his negotiations with Princesschalle ended. Marco tries to warn Guido from reckless decisions and then reports that Princivalle, having learned of his fate (they are going to accuse him of treason and executed), offers military assistance to the city of Pisa or promises to send three hundred carts with ammunition and food. But Princessville sets the condition (Marco hardly forces himself to pronounce it) so that as a sign of humility, as a sign of victory, she comes to him alone, "she comes completely naked, so that only a cloak serves her as a cover" - Guido's wife, Giovanna. Guido is outraged. He is ready to die himself and destroy the city, but it turns out that Giovanna already knows from Marco about the condition for saving Pisa and is ready to sacrifice herself Guido is trying to prevent his wife. Realizing that it was useless, he parted with her coldly.

In her camp near Pisa, in a tent where weapons, furs are lying around in disorder, chests with jewels and shiny fabrics are standing, Princivally is waiting for her fate to be decided: if his offer is rejected, Marco should return, if accepted, he’ll light up on the bell tower in the city a fire announcing the arrival of Giovanna, Monna Bath, the woman that Princess Primal loves since childhood. The signal light comes on. Princess is rejoicing. But before the appearance of Monna Vanna Princivalle, a meeting with Trivulzio, the commissioner of the Florentine Republic, is ahead. Trivulzio assures Princivall of his sincere affection and warns of the machinations of ill-wishers. He calls on the commander to immediately take Pisa with an attack, so that, having returned as a triumphant to Florence, to attract to his side those who are hostile towards him. Princivalle exposes the duplicity of Trivulcio, showing him his own denunciations, as a result of which Princivalle was supposed to die, because the people of Florence idolized him and could follow him if Princulla had the idea to rebel against her superiors. Feeling that he was exposed, Trivulzio throws himself with a dagger on Princechall, who manages to deflect the blow, and the dagger only touches his face. Princesschalle does not understand how to destroy a person on one suspicion only, out of fear of imaginary danger. At the same time, he respects the loyalty of Trivulcio to his native Florence, Princivalle orders to take Trivulzio away, but warns that no one will touch him with his finger. Vedio, adjutant of Princivally, bandages his wound. A shot is heard in the distance. Princesschalle is worried: what if they shot Monna Bath? Vedio leaves to find out and, returning, calls to Princivalle. Then he disappears, and Monna Bath appears in the tent. She is indeed slightly injured in the shoulder, but refuses to dress the wound. Princesschalle shows Bath how carts with provisions and ammunition are sent to him to pay for her arrival in Pisa.

Having set Bath on his bed, Princesschalle tells her about her love. The bath is struck by the power of feeling. She did not immediately recognize in the commander of the enemy army the blond boy Janello, with whom she played in childhood. Father took Janello to Africa. After long wanderings in the desert, after Turkish and Spanish captivity, he returns to his hometown and finds out that Giovanna married the most powerful and wealthy man in Pisa. He had nothing to offer her. Gianello becomes a hired commander, participates in various wars, his name becomes glorious, and now the case brings him under the walls of Pisa ... Giovanna reproaches him with indecision. “Do not flatter yourself, I do not love you ...” she says to Princivalle. “And at the same time, the very soul of love in me is writhing, grumbling, and indignant when I think that a person who loved me so passionately as I could love him would suddenly lack the courage in love!” When asked if she loves Guido, Giovanna replies that she is happy with him - how happy can be a person who has abandoned reckless dreams ...

It’s embarrassing for Bath that for her sake Princivalle recklessly at stake her future, her fame, her very life, and he explains to her that he did not sacrifice anything for her: he is a mercenary and faithful only as long as you are faithful to him (“Be my motherland , I wouldn’t cheat on her for the sake of fiery love, ”he remarks). Vedio warns Princivalle about the appearance of the Florentines in the camp, ready to arrest him. Giovanna, in order to save Princesschalle, invites him to leave with her in Pisa. Above the city towards which Bath and Princesschille are heading, there is a glow of festive lights. The bathtub is happy and grateful to Princessville. She kisses his forehead.

In Pisa, in his palace, Guido suffers from shame, from humiliation. He no longer wants to see his father pushing Giovanna to the victim. And he is going to forgive her, but only when her rapist is killed. “It’s perhaps impossible to completely forget her misconduct, but he can go so far into the past that he will not find his jealousy ...” Marco is ready to leave the city, he only wants to see Giovanna meet with Guido. The cheers of the crowd are heard: “Our Monna Bath!”, “Glory to Monne Bath!” Marco, accompanied by Borso and Torello, goes to the terrace, Guido is left alone. Marco's eyes are unable to discern where Giovanna is, and Borso tells him of her triumphal approach. Next to her is an unknown person whose face is hidden by a blindfold. Marco hugs Giovanna. Guido appears. The bath wants to talk to him, rush into his arms, but he abruptly stops and pushes Giovanna. He drives the crowd from the walls of his palace, then tries to remove the bandage from the face of Princivalle to find out who it is. The bath rises between them. She reveals to Guido that this is her savior and calls his name - Princessville. Guido decides that Giovanna brought Princess to Pisa to avenge him. Having run out onto the terrace, Guido publicly screams that the enemy has been caught. Now he wants the crowd to gather. Guido is eager to hear Bath's tale of how she managed to lure Princesschalle. Bath urges her husband to believe her and says that Princesschille did not touch her. But the common sense of Guido does not allow him to believe his wife. He turns to the crowd asking if anyone believes Giovanna? Only Marco believes her. And Guido confronts Giovanna with a choice: either she confesses that Princivalle possessed her, or if she insists that he did not touch her, he will be executed. Then Bath, in order to save Princesschalle, lies that he possessed her, that she lured him into the city with kisses (while she passionately kisses Princesschille, whispers words of love to him and conjures him to remain silent). She demands that she be given the key to Princhalle's dungeon, and Guido promises that the guards will bring her the key now.

Marco understands and accepts the high deception of Monna Bath. Guido is happy, for him the past is a heavy dream. “Oh yes, you're right,” Bath answers him, “it was a heavy dream ... And now - now the light will begin ...”