Gennaro, the son of Lucretia Borgia, lives unaware of the identity of his mother, who has married the Duke of Ferrara. After Lucretia's brother is killed by five conspirators, the fathers of Gennaro's dearest friends, Lucretia tortures the old men to death. Later, Gennaro and his companions journey to Lucretia's domain, and she sees her son for the first time. The Duke, who believes him to be her lover, poisons him, but Lucretia administers the antidote in time and saves his life. Then she schemes to poison her sons' five friends for their fathers' mistake. She succeeds in poisoning them all at a dinner at which Gennaro is an uninvited guest. In dismay, she pleads for him to take the antidote, but he refuses and in a fury avenges his friends by stabbing Lucretia. As he lies dying, he learns that she was his mother.