The ancient realm of Byzantium, home of Byzantine culture, has fallen to the forces of the Turkish sultan Maometto II, and the Vene­tian city Negroponte is under siege. On the Byzantine side, Calbo urges the commander Paolo Erisso to continue defending the city; Condulmiero urges surrender. Erisso, who has arranged the engagement of his daughter Anna to Calbo, does not know she is in love with a nobleman of Corinth.

Turks overrun Negroponte’s defenders, toking Erisso and Calbo as prisoners. The sultan Maometto arrives, recognizing Anno as his clandestine lover. The stunning truth of the re­alization horrifies both Erisso and Anno, who threatens suicide unless Maometto frees Erisso and Calbo. Maometto agrees, promising Anna a life of love and luxury.

Still overwhelmed by Maometto’s revelations and her homeland’s plight, Anna rejects his ardent declarations of love. He leaves to press the attack on Negroponte, entrusting her with his seal of imperial authority as a guarantee of her safety while he is gone. But Anno joins Calbo and her father in hiding, overcoming Erisso’s accusations of disloyalty by giving him Maometto’s seal. Now free to emerge from hiding, Erisso conducts Calbo’s marriage to his daughter in the shadow of her mother’s tomb, and the two men leave to face the Turks in combat.

Temporarily at a distance from the conflicting men in her life, Anna receives good and bad news: Erisso hos defeated Maometto and suc­cessfully defended Negroponte, but Anna is now in danger from Maometto, who will un­doubtedly want to avenge her betrayal. Con­fronted by Maometto and his troops at the very spot where she was married, Anna reveals the details of her deception and stabs herself, dying on her mother’s tomb.