Byzantium, 6th century AD, and Emperor Michael II convenes a meeting to elect a wife for his regent, Theophilus, who, although charmed by the nineteen-year-old princess Cassiane, ultimately chooses Theodora. His general, Acillas, who is in love with Cassiane, is appointed commander of the forces in Sicily and, before leaving, obtains Cassiane's promise that she will marry him, even though she does not love him. In Sicily, the noblewoman Irene falls in love with him, but he cannot forget Cassiane. Theophilus secretly meets Cassiane, who has begun to write religious hymns. Rumors that Cassiane is the regent's mistress reach the ears of Aquila, who rushes to Constantinople with the intention of killing Theophilus, but is himself killed by the palace guards. After this, Cassiane locks herself away in a monastery and devotes herself to hymnography.

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