Gismonda is the widow of the Duke of Athens, and the mother of his child, a five-year-old boy named Francesco. Gismonda is the ruling power of the duchy as regent for her son, an absolute monarch. She is surrounded by a flattering court, among whom is a Venetian, named Zaccaria Franco, who loved the Duchess before she married the Duke of Athens. He is seemingly one of her strongest supporters, but is actually trying to seize power for himself. Zaccaria is in league with the Turks, who support him in his plotting against the duchy. Gismonda’s young son Francesco, the heir to the duchy, stands between Zaccaria and his ambitions. So Zaccaria has conspired with an accomplice, Gregorez, to murder the boy.