The handsome but brutal tribune M. Vinicius, returning to Rome from service in the east, falls in love with "Lygia", a hostage daughter of the Lygian king, who is being raised in the house of Aulus Plautius (a general of British fame), and his wife Pomponia Graecina, who is secretly a Christian. Petronius uses his influence with Nero to have Lygia seized from the Plautius' and given to Vinicius; but the plan misfires when Caesar, during her brief custody on the Palatine (in which she meets Acte), invites her to a riotous feast, where Lygia, inculcated with Christianity by Pomponia Graecina, is horrified by Vinicius' drunken advances, and the degeneracy of the Roman court. She commands Ursus (her Lygian bodyguard, and also a convert) to organize a band of Christians to waylay her chariot while she is being conveyed the following day from the Palatine to Vinicius' house; the plan succeeds, and Lygia disappears.
Vinicius is now driven to distraction with the thwarting of his obsessive desire; Petronius, taking pity on him, secures him the services of the cadging Greek philosopher Chilo Chilonis; from the sign of a fish which Lygia had drawn Vinicius in the house of Plautius Chilo discovers that Lygia is Christian; and since a vigilant watch on the gates has revealed that she is still in the city, Chilo undertakes to disguise as a Christian to worm out the secret of her hiding-place. Hope revives when Chilo recognizes Ursus in Urban, a common Christian laborer. When he learns that the entire Christian community in the city is to meet at night in Ostrienum outside the city walls, to hear Peter the Apostle (lately arrived from Galilee), Vinicius insists on accompanying Chilo to the event in disguise, hoping to see Lygia; although momentarily impressed by Peter's recollections of Christ, Vinicius forgets all when he spots Lygia in the crowd; together with Chilo and the powerful athlete Croton, originally brought along in case of danger, he follows Lygia and Ursus from the meeting to a plebeian insula in the trans-Tiber region of the city; he and Croton enter the building to retrieve Lygia, but Ursus strangles Croton to death and nearly kills Vinicius, sparing him only at Lygia's intercession; the cowardly Chilo flees.
Here Vinicius is magnanimously nursed to health by Lygia and her fellow Christians, who to his immense surprise, have forgiven him all; he is further shocked when, on his summoning Chilo (by agreement with the Christians) to communicate to his household that the cause of his disappearance is a sudden trip to Beneventum, it emerges that Glaucus, the Christian doctor who is attending Vinicius, had been betrayed by Chilo to bandits during a previous period of his unscrupulous adventures; whereupon with Peter's approval Glaucus forgives him all. Meanwhile, when Lygia realizes, while acting as his nurse, that she is herself deeply in love with Vinicius, she confesses to Peter, who while affirming that her love is not sinful, says she cannot marry Vinicius as long as he is not a Christian. Lygia changes her residence, vanishing a second time.
