Three hundred years after the death of Christ, when Roman Christians are forced to practice their religion in underground caves, a slave named Vibio is purchased by Fabio, a Roman patrician. For his refusal to fight against a professional wrestler, Vibio is whipped by the patrician's daughter Fabiola. Vibio is defended by Fabiola's gentle cousin, Agnese, and the emperor's favorite tribune, Sebastian, both of whom are secretly Christians. When the hiding place of the Christians is discovered, Fabiola agrees to help them, although she does not share their faith. She falls in love with Vibio and remains with him until word arrives that Sebastian's religion has been discovered and he has been sentenced to death. Fabiola tries to intervene in his behalf, but she is imprisoned and Sebastian is executed. Vibio then rallies the Christians, and they break into the sewers beneath the jail where hundreds of Christians, including Agnese, are waiting to be dragged into the arena. Before many of them can be saved, however, they are burned on crosses or slaughtered by condemned criminals. As Vibio and his men forcibly enter the arena and witness the gory spectacle, they drop their swords in horror. The massacre affects even the blood-thirsty crowd, and they prevail upon the emperor to spare Vibio, Fabiola, and the remaining Christians.