Pompeii, A.D. 79. Athenian nobleman Glaucus arrives in the bustling and gaudy Roman town and quickly falls in love with the beautiful Greek Ione. Ione's former guardian, the malevolent Egyptian sorcerer Arbaces, has designs on Ione and sets out to destroy their budding happiness. Arbaces has already ruined Ione's sensitive brother Apaecides by luring him to join the vice-ridden priesthood of Isis.
The blind slave Nydia is rescued from her abusive owners by Glaucus, for whom she secretly pines. Arbaces horrifies Ione by declaring his love for her, and flying into a rage when she refuses him. Glaucus and Apaecides rescue her from his grip, but Arbaces is struck down by an earthquake, a sign of Vesuvius' coming eruption.
Glaucus and Ione exult in their love, much to Nydia's torment, while Apaecides finds a new religion in Christianity.
Nydia unwittingly helps Julia, a rich young woman who has eyes for Glaucus, obtain a love potion from Arbaces to win Glaucus's love. But the love potion is really a poison that will turn Glaucus mad. Nydia steals the potion and administers it; Glaucus drinks only a small amount and begins raving wildly.
Apaecides and Olinthus, an early Christian, determine to publicly reveal the deception of the cult of Isis. Arbaces, recovered from his wounds, overhears and stabs Apaecides to death; he then pins the crime on Glaucus, who has stumbled onto the scene.
Arbaces has himself declared the legal guardian of Ione, who is convinced that Arbaces is her brother's murderer, and imprisons her at his mansion. He also imprisons Nydia, who discovers that there is an eyewitness to the murder who can prove Glaucus's innocence – the priest Calenus, who is yet a third prisoner of Arbaces. She smuggles a letter to Glaucus's friend Sallust, begging him to rescue them.
Glaucus is convicted of murder, Olinthus of heresy, and their sentence is to be fed to wild cats in the amphitheater. All Pompeii gathers in the amphitheater for the bloody gladiatorial games. Just as Glaucus is led into the arena with the lion – who, by a miracle, spares his life and returns to his cage – Sallust bursts into the arena and reveals Arbaces' plot. The crowd demands that Arbaces be thrown to the lion, but it is too late: Vesuvius begins to erupt. Ash and stone rain down, causing mass panic.
Arbaces grabs Ione in the chaos but is killed by a lightning strike. Nydia leads Glaucus, Sallust, and Ione to safety on a ship in the Bay of Naples. The next morning she commits suicide by quietly slipping into the sea; death is preferable to the agony of her unrequited love for Glaucus.
Ten years pass, and Glaucus writes to Sallust, now living in Rome, of his and Ione's happiness in Athens. They have built Nydia a tomb and adopted Christianity.

Question: Who is an eyewitness to Arbaces stabbing Apaecides to death?
Answer: The priest Calenus.
Alternative answer: Glaucus