A Roman commander falls for a Christian slave girl as Nero intensifies persecution of the new religion.

In 64 A.D. commanding officer Marcus Vinicius returns to Rome after three years abroad waging battle for emperor Nero, a tyrant who believes he is a gifted divinity. Nero's most trusted advisor Petronius, who is also Vinicius' uncle, informs his nephew that Nero has recently murdered his wife and mother and married a slave named Poppaea, and that the disgruntled Roman Senate is making plans to replace Nero with General Galba of Tuscany. While visiting the home of retired General Plautius, Vinicius flirts with a woman he assumes is a household slave, but soon discovers that she is Plautius' daughter Lygia, who rebuffs his crass advances. Over dinner, when Vinicius eagerly describes the defeat of Rome's enemies, Lygia expresses her disgust with the brutalities of war. Plautius explains that Lygia was once a princess who was made a slave during his military campaign against her people. Plautius and his wife Pomponia adopted Lygia in attempt to make amends for her suffering. Later, when family friend Paul philosophizes about peace, Vinicius insists that Lygia is too lovely to worry about such trivial teachings and leaves. Paul then tells the family that the apostle Peter, who spoke with their savior Jesus Christ before his death, will arrive in Rome shortly. Later that night, after Vinicius invites Lygia to a feast celebrating the legions' triumph, she reveals she is attracted to Vinicius, but admits his tales of conquest disturb her and refuses him again. After he leaves, Lygia prays for Vinicius' conversion to Christianity. During the military parade the following day, Petronius suggests to Nero that he buy Vinicius a slave as a sign of gratitude. Nero orders Lygia taken and given to Vinicius. While Vinicius tries to interest the sullen Lygia in the palace festivities that night, the conniving Poppaea jealously spies on them. After Nero arrogantly sings amateurish lyrics while accompanying himself on the lyre, Petronius suggests Nero must improve his verses to reflect his "true genius." Nero then exclaims that he might burn the city just to inspire him to create a great epic. Later, while Lygia is being escorted to Vinicius' quarters, her guard, the giant Ursus, attacks the escorts, allowing Lygia to escape. The next day, when Vinicius seeks Petronius' help in locating Lygia, Petronius reveals that Paul is a frequent visitor at Plautius' home. Petronius explains that Lygia, like Paul, is a Christian, members of a secret sect that worships Christ, an opponent of the state who, although crucified, is still of political concern to both Nero and the Senate. Petronius sends his friend to Chilo, a soothsayer, who leads Vinicius to a Christian rite held in a cave that evening. During the ceremony, Peter describes his first meeting with Jesus at Galilee, where the savior miraculously filled their empty fishing nets with catch. Peter continues with the story of how he and eleven other apostles followed Jesus, who was crucified at Calvary. Soon after, Jesus appeared before the apostles, forgave them for their sins and bade them to follow the Ten Commandments and abstain from violence. After the meeting, Vinicius and his guard Croton follow Lygia, but Ursus kills Croton and knocks out Vinicius to protect Lygia. He then carries Vinicius to a hideout, where Lygia tends to his wounds. Vinicius asks her to marry him and offers to fill their home with grand sculptures celebrating her god, but Lygia says she has no need of expensive gestures because she carries the image of Christ in her heart. Driven by jealousy, Vinicius demands that Lygia choose between her faith and him. When she chooses Christ, Vinicius leaves for Antioch, where Poppaea, having heard about his failure with Lygia, tries to entice him into an affair. That afternoon, Nero, surrounded by his council, announces that he killed his mother and past wife to experience a great sacrifice and thereby inspire his "new creative vision." He then unveils a sprawling architectural model of a city called "Neropolis," which will replace Rome. When Petronius asks what will become of the existing city, Nero announces that he has set fire to Rome. Fearing for Lygia's life, Vinicius steals a chariot and races to Rome, where buildings are tumbling down and fires spill out over thousands of citizens. Opening a sewer grate, Vinicius leads a crowd to the city's edge, where he spots Lygia. Petronian guards, following Nero's orders, block the exits out of the city, but Vinicius fights the commanding officer and orders the troops to break ranks, thus freeing thousands of people from imminent death. Soon, the citizen mob reaches the palace at Antioch prepared to kill Nero for his incendiary act. Desperate to find a scapegoat, Nero orders his commanding officer Tigellinus to take the blame, but Tigellinus threatens to turn his legions against Nero. When Poppaea suggests sacrificing the Christians, Nero agrees, but Petronius warns that the Christians will then become martyrs. The next morning, while Petronius signs a petition presented to him by Vinicius requesting Galba replace Nero, he warns his nephew that Poppaea has issued a warrant for his arrest and that Christians are being imprisoned for setting fire to Rome. Vinicius searches for Lygia in the prisons, where he is thrown into a cell with her and her parents. Learning that they will soon be fed to the lions, the Christian prisoners demand to know why God has deserted them; however, Plautius and Pomponia encourage the crowd to be courageous and have faith in God. Meanwhile, travleing toward Greece on the road outside Rome, Peter witnesses the skies filling with light as God, speaking through his fellow traveler, the young orphan Nazarius, announces, "My people in Rome have need of thee," thus causing Peter to return to Rome. That night, Petronius holds a dinner for his friends and announces he is freeing his slaves, including Eunice, to whom he has devoted his love. Denouncing his cynical wit, and believing that a better life awaits him after death, Petronius orders one of his servants to slit his wrist. Eunice, distraught by her lover's act, slits her wrist as well. As they lay dying at the head of the table, Petronius dictates a letter to Nero in which he implores his leader not to "mutilate the arts" with his "mediocre performances," and admonishes him to "brutalize the people but do not bore them as you have bored your friend." When the letter is delivered, Nero seethes at his advisor's words. Later, at the Roman arena, Nero and Poppaea are awaiting the first sacrifice of the Christians, when Peter enters among the spectators and tells the faithful that they are blessed for dying in the name of Christ. His words prompt the victims to sing fearlessly as the lions attack them, infuriating Nero. In a cell that evening, Lygia asks Peter to marry her and Vinicius, who is beginning to understand their faith. Soon after, Peter is crucified along with Plautius, who publicly accuses Nero of setting fire to Rome. The next day, Vinicius is forced to watch beside Poppaea as Ursus guards Lygia from a charging Brahma bull in the arena. When Vinicius calls out to Christ to give the guard strength, Ursus wrestles the beast to the ground and kills it. As the crowd and council demand that Lygia and Ursus be spared, Vinicius announces to the public that Galba will soon take over as emperor of Rome. Nero flees the arena to his palace, which is surrounded by throngs of irate Roman citizens. Accusing Poppaea of encouraging him to make martyrs of the Christians and thus cause his downfall, he chokes his wife to death then locks himself in his room. Slave Acte is waiting there and hands her master a dagger, telling him to kill himself like an emperor. A coward to the end, Nero begs her to help him plunge the knife into his breast. In the following days, as Galba's troops march into Rome, Vinicius admits that all dynasties are destined to fail and observes that hope resides in one faith that will unite the world. Soon after on the road out of Rome, Nazarius shows Lygia, Vinicius and Ursus the blessed spot where God spoke through him to Peter, which is marked by Peter's upright cane covered with blooming vines.