Act 1
Arianna, the widow of the emperor of Byzantium, marries Anastasio and makes him the new regent.

The tyrant Vitaliano claims Arianna and her empire and captures Arianna in the war that ensures.

Vitaliano’s brother, Andronico, is in love with Anastasio’s sister, Eufemia, and infiltrates the court dressed as a woman named Flavia, in order to abduct her.

Giustino, is ploughing his fields, but dreaming of being a war hero. The goddess Fortuna appears in a dream and promises him a glorious future.

The next morning Giustino feels as strong as an ox. He sees a young woman running from a bear and saves her: it is Eufemia, and she immediately falls in love with him.

Eufemia  takes her beloved farmer to the palace, where they are given a warm reception.

Meanwhile, the captive Arianna refuses to surrender to Vitaliano. He decides to feed her to a voracious sea monster.

Act 2
Emperor Anastasio is determined to free Arianna; Giustino offers to go with him as his knight.

After a shipwreck, the two men wash ashore on the same island where Arianna is to be fed to the monster.

The chained Arianna is liberated by Giustino after a battle between the forces of Vitaliano and Anastasio, with Giustino leading the troops. 

Act 3
Giustino takes Vitaliano prisoner in battle, then rescues Eufemia from the clutches of Andronico, who has thrown away his disguise and is about to take her by force.

Eufemia is once again happy with her farmer hero and love blossoms between them.

Arianna wishes to reward Giustino for his valour and as a token of her gratitude gives him Vitaliano’s jewelled belt.

Egged on by his general Amanzio, Anastasio, becomes convinced that Arianna is in love with Giustino and that his power is threatened.In a jealous rage, he repudiates his wife and condemns Giustino.

Giustino manages to set himself free, but is seriously injured. Amanzio seizes the throne for himself, with the help of the turncoat Polimante.

Vitaliano and Andronico escape from custody. They come across the unconscious Giustino and Vitaliano sees his chance to finally kill his enemy.

Vitaliano’s dead father appears as a ghostly apparition telling him that Giustino is his brother.

Vitaliano joins forces with Giustino to overthrow Amanzio and restore Anastasio to the throne.Arianna forgives Anastasio.

Anastasio declares Giustino his co-ruler and gives him Eufemia’s hand in marriage;

Giustino seeks clemency for his brothers Vitaliano and Andronico;

And Fortune crowns the virtuous Giustino with honour and glory.