ACT I: Hall in the emperor's palace. Irene and the populace greet the victor Belisarius. Antonina hates her husband because Proclus, the slave of Belisarius, has confessed on his deathbed, that upon command of his master he had exposed her son on the shore of the ocean, thus causing his death. The Emperor Justinian greets his commander and grants his prayer for the release of the prisoners. The captive, Alamiro, who adores Belisarius, refuses to leave him. (Recitative and duet: "What do I see, does Alamiro reject my gift?") The general adopts him in place of his long lost son. Irene congratulates her father, but Antonina has already begun her work of hate, by traducing Belisarius to Justinian, and the innocent man is accused of high treason and thrown into prison on the evidence of his wife

ACT II: Before the prison. Alamiro and his friends lament the fate of Belisarius. His eyes have been put out by his enemies, who have falsely construed and disobeyed the commands of the emperor. Alamiro swears vengeance. (Aria: "Tremble, Byzantia, I will repay.") Irene clad as a youth arrives to act as guide to her father, who is about to be released from prison. (Duet: "Oh thou, who in terrible darkness.")

ACT III:In the mountains. As the clang of weapons is heard Irene leads Belisarius to a cave for safety. Alamiro now leads the army of the Alannae against Byzantium to avenge Belisarius. Belisarius confronts him and recognises him as his son through an amulet. At his father's request, the son leaves the ranks of the enemies of Byzantium, and the Alannae, now under the command of Ottavio, march to Byzantium, having no fear, as the emperor's army is bereft of its leader. Change of scene: Hall in Byzantium. Antonina, in remorse, tells the emperor that her testimony against Belisarius was false. Irene approaches with news of the victory and informs Antonina that Alamiro is her son, and that it was the slave, not Belisarius, who had planned his death. Meanwhile the blind Belisarius has led the Byzantine army and defeated the Alann, who had threatened Byzantium, but an arrow has mortally wounded him. He is carried in dying, and the sorrowing emperor promises to be a father to Alamiro and Irene.