Ottone, German king and now Emperor is about to marry a Byzantine princess. When the ship carrying her to Italy is attacked by ‘Saracen’ pirates, he pursues them and takes their leader prisoner. His promised bride, Teofane, proceeds unmolested to meet Ottone in Rome.

In Rome a rival prince (Adelberto) and his powerful mother (Gismonda) stage a coup d’etat. Adelberto claims the throne, and tries to claim Ottone’s bride as well. Teofane is confused: this man does not resemble the man in the portrait sent to her.

Ottone’s defiant prisoner refuses to disclose his name, or show obedience. Matilda, Ottone’s sister, arrives at camp and reveals to him Gismonda’s seizure of Rome; although betrothed to Adelberto, Matilda knows that he has used and rejected her, so she undertakes to lead an army against him. Ottone praises her loyalty.

At court, Teofane is baffled by the tough-minded Gismonda (who pretends to be the mother of Ottone, rather than a rebel); as her marriage to ‘Ottone’ is about to take place, the real Ottone storms the city, and Adelberto rushes to defend himself. Teofane is left in lonely confusion.

Ottone returns to court with two captives: the silent pirate Emireno and the defiant Adelberto, who complains that he has not enjoyed Teofane before death.

Confronting Adelberto with his treachery – both personal and political – Matilda’s infatuation overwhelms her. She confers with Gismonda, who rejects any idea of begging Ottone for mercy – though the mother’s belligerence cools as she thinks of the fate of her son.

Just as Ottone and Teofane are at last to meet, Matilda intervenes; she pleads for mercy for Adelberto. Ottone consoles her with an embrace, but rejects her plea – and this is observed by Teofane, who misinterprets their intimacy. As Matilda leaves with a curse, Teofane also flees.

Emireno and Adelberto have escaped prison by Matilda’s contrivance. In a cave by the sea at night they prepare to flee. There Teofane by chance has strayed, yearning to return to the east. Adelberto abducts her, and Emireno makes good their escape.

Gismonda, concealed, has witnessed their flight. Matilda arrives moments too late. Together they call for night to hide their deed, and the desperate fugitives. Matilda hurries back to court when Ottone, too, is drawn to this sea cave, in search of Teofane.

Gismonda taunts Ottone: his rival Adelberto is free, has taken Teofane, and will return with an army.

A storm has forced the escaping party to land on a desolate shore. When Adelberto goes to reconnoitre, Teofane claims that either Ottone or her brother, a Byzantine prince, will avenge her abduction. Questioned, she reveals that her brother was ousted by a usurper, but as the usurper has now been defeated she expects her brother’s return. Emireno recognises his lost sister, and goes to embrace her – which she first interprets as an assault. On Adelberto’s return, Emireno overwhelms him and pledges to return to Rome with his captive and his sister.

Learning that Adelberto has taken Teofane with him, Matilda is furious. Gismonda exults in revealing to Ottone that Matilda was complicit in the escape – so that he is surrounded by betrayal. Matilda swears to make amends by killing Adelberto.

Emireno returns with the captive Adelberto. Matilda is about to kill him when Adelberto’s miserable confession softens her heart. Gismonda scorns her weakness, takes the dagger, and makes to kill herself. Teofane stops this, calling for clemency.

Startled by love, Ottone agrees – just as Adelberto, wakened by Matilda’s compassion, vows to love her.

Brother and sister, husbands and wives, mother and son, ruler and subjects pledge a new faith, and a new love, strengthened by suffering.