A childhood friend enlists Hercules to protect the Roman Emperor Gordiano, who is in danger from his own mutinous Praetorian Guards. By the time the hero reaches Italy and the temporary Roman capital of Ravenna, the guards have already murdered the mild and scholarly Emperor, replacing him with their chief officer Filippo Afro. Gordiano's daughter Ulpia faces the prospect of marrying Filippo's worthless son Rezio, linking the usurper to the imperial family. Hercules undertakes to set the situation right, rescuing Ulpia and helping the great Roman general Lucio Trajano Decio seize the throne.