Based on the novel by Henryk Sienkiewicz, a Roman conqueror falls in love with a Christian in the final years of Nero’s Rome.
In 64 CE the general Marcus Vinicius (Robert Taylor) returns to Rome and is ordered to camp outside the city. Emperor Nero (Peter Ustinov) tells Vinicius to wait a few hours for a triumphant entrance. Vinicius is advised by his uncle Petronius (Leo Genn). Vinicius meets beautiful Lygia (Deborah Kerr) while staying with retired general Plautius (Felix Aylmer). Paul (Abraham Sofaer) arrives from Antioch. Vinicius woos Lygia, who does not like his conquests. Strong Ursus (Buddy Baer) guards her. Lygia prays.
Petronius persuades Nero to appear in public for the triumph. Petronius presents his slave Eunice (Marina Berti) to Vinicius; she loves Petronius, and Vinicius rejects her. Vinicius wants the hostage Lygia. A soldier takes Lygia to Nero’s palace and Vinicius. Nero sings, and two men fight. Nero gives Lygia to Vinicius.
Vinicius learns that Lygia is a Christian and goes to a secret meeting. Paul baptizes a man and introduces Peter (Finlay Currie), who preaches about Jesus. Vinicius sees Lygia there and follows her. Ursus knocks out Vinicius and kills the gladiator Croton. Lygia nurses Vinicius, and Ursus asks Vinicius for forgiveness. Vinicius tells Lygia that she can go home, but she kisses him and agrees to marry him. Paul asks Vinicius to free his slaves. Lygia chooses Christ over Vinicius, who leaves.
Nero gets angry at Acte (Rosalie Crutchley) and banishes her. Vinicius goes to Empress Poppaea (Patricia Laffan), who kisses him. Nero explains why he executed his mother and wife. He plans to rebuild Rome, and Tigellinus (Ralph Truman) says Rome is burning.
Vinicius rushes to save Lygia. Nero sings while a mob throngs the palace. Nero blames Tigellinus, but Poppaea suggests that he blame the Christians. Petronius says that history will condemn Nero. Vinicius tells Lygia that he must stop Nero. Vinicius gets Petronius to sign his letter to Galba. Vinicius learns that Plautius has been arrested, and Tigellinus arrests Vinicius. Lygia says they must face the lions.
Peter sees a vision in a tree, and the boy Nazarius (Peter Miles) says his people in Rome need him. Petronius frees Eunice and has a physician cut his wrist. Eunice cuts her wrist too. Petronius dictates a letter to Nero condemning his boring verses. Nero learns that Petronius is dead and reads his letter.
Nero and Poppaea preside in the arena where Christians await the lions. Peter speaks and is arrested. Christians sing, and lions are released. Poppaea tells Nero to hold back Vinicius and Lygia. In the cell Lygia wants to marry Vinicius, and Peter weds them. Peter is crucified upside down. In the arena crucified Plautius accuses Nero. Poppaea has Vinicius watch Ursus protect Lygia from a bull. Ursus breaks the bull’s neck. Vinicius gets free and joins Lygia. Nero puts his thumb down, but soldiers loyal to Vinicius rescue him. He announces that Galba is coming to replace Nero.
In the palace Poppaea tells Nero that the praetorian guard has revolted. Nero strangles her. People enter the palace. Acte gives Nero a knife and helps stab him. In the final scene on the road Nazarius shows Vinicius and Lygia the place where Christ spoke to Peter.
This historic epic contrasts the brutality of Roman power to the gentle love of persecuted Christian martyrs. When an immature person is given the extraordinary authority of Roman emperor, the results are disastrous. Yet the spiritual power of the poor who know how to love spreads its message and transforms more and more people.