Martyr of the Catacombs tells the story of Christians during the great persecution under the Roman Emperor Decius. Marcellus is a successful, young Roman Army officer, chosen by the Emperor for that highest of advancements, assignment to the Praetorian Guard. Upon his arrival in Rome, he witnesses a spectacle in the Coliseum. A famous gladiator Macer defeats all opponents, but at the end he announces that he will not kill a fellow gladiator because he is a Christian. He declares his faith in Jesus Christ, infuriating the mob, and allows his defeated opponent to slay him. As Marcellus ponders upon what he has seen, his friend Lucullus brings him word from the Emperor as to why he has been brought to Rome. Marcellus is to lead a group of soldiers from the Praetorian Guard down into the catacombs to hunt these Christians out and bring them to punishment. While on a scouting mission, in his effort to gather intelligence on them, Marcellus meets several Christians, becomes closer to them and to Christian faith which he eventually accepts, becoming a fugitive and an outlaw.