A blacksmith becomes a gladiator, captures slaves, and steals horses to gain money; but his adopted son is influenced by a life-saving encounter with Jesus and helps slaves escape during the volcanic eruption.
The blacksmith Marcus (Preston Foster) helps soldiers restrain a gladiator. He is happy with his wife and child and does not wish to fight until they are injured by horses and he needs money for the doctor and taxes. Marcus kills for coins and learns his wife and son died. He fights for money and adopts the son of a man he killed. Marcus buys the Greek tutor Leaster (Wyrley Birch) for this boy Flavius (David Holt). Marcus loses a fight, ending his gladiator career. Next he captures slaves in Libya so he can buy horses in Judea. A soothsayer tells him to take his son, because he will meet the "greatest man." Marcus rejects a man in a stable and goes to Pontius Pilate (Basil Rathbone), who suggests he rob horses from the king of Amman. Pilate sends him prisoners led by Burbix (Alan Hale) to do that. Marcus finds Flavius in a coma; but he takes him to Jesus, who heals him. Marcus returns to share the loot with Pilate, who has just washed his hands of a difficult case he regrets. Marcus refuses to try to save Jesus and leaves with his loot.
Years later Marcus is head of the arena and is rich. He gives a sword to Flavius (John Wood), who objects to arena killing. Flavius is helping runaway slaves to escape to an island. He meets Clodia (Dorothy Wilson) and tries to remember the man who cured him. The prefect (Louis Calhern) tells Marcus that slaves are escaping. Marcus welcomes Pilate, and they dine with the prefect, who has learned where the slaves are hiding. Marcus wants Flavius to go to Rome with Pilate; but he refuses, because he wants to help the persecuted. Flavius warns the runaways, and they learn he is the son of the cruel Marcus, who would make them fight in the arena. Burbix tells Marcus the runaways were captured, and Leaster tells him Flavius is with them. Marcus fails to get Flavius released, as the prefect starts the games. During the arena fighting Mount Vesuvius erupts, and people panic. Earthquakes cause destruction. Marcus rescues the son of the warder who would not release Flavius. Marcus throws away his money to help the wounded. Lava drives people into the sea. Marcus lets Flavius and his friends escape on a boat while he holds back the prefect's soldiers and is killed. Marcus then sees Jesus.
Using the title and setting but not the story of Bulwer-Lytton's novel, Christian values are contrasted to the brutality of gladiators and slaves. By making money his god Marcus rejects the former for the latter until the end.