The goddess Fortuna promises the farmhand Giustino honour, glory and wealth if he has the courage to go out into the world. On his way he protects a woman from a bear, ends up in Byzantium where Emperor Anastasio reigns. The woman Giustino saved is Leocasta, the Emperor’s sister. The Emperor’s wife Arianna has just been kidnapped by the tyrant Vitaliano and he is being threatened by his general Amanzio who wants to be emperor himself. Giustino rescues the Empress, but the thankful behaviour of Arianna causes the jealousy of the Emperor. An intrigue of Amanzio makes him sentence Giustino to death. But Leocasta succeeds in freeing Giustino. Meanwhile Vitaliano plans to take revenge and is about to kill Giustino when he slumbers, but in this moment the two men realise that they are brothers, and together they hurry to Byzantium where they are only just in time to prevent the execution of the Emperor who has been overthrown by Amanzio. All’s well that ends well: Giustino arries Leocasta and is app inted co-regent.