In the wealthy and powerful city of Troy, the pregnant queen Hecuba experiences a prophetic dream which distresses her. When consulted, a priestess of the Great Goddess tells Hecuba and her husband, King Priam, that the dream indicates she will birth a son who will bring destruction to Troy. Priam declares that this boy must be exposed to death, but upon his birth three days later, Priam agrees to Hecuba's pleas and has him fostered by a shepherd on the slopes of Mount Ida. Priam names the boy Alexandros (later Paris), and names his twin sister Alexandra, whom Hecuba keeps and decides to call Kassandra.
While visiting the temple of Apollo with her mother, six-year-old Kassandra experiences a vision of the god telling her she is to become "his priestess". In the following years, Kassandra experiences further visions. When she is twelve, Kassandra sees a vision of Paris, who is now a shepherd. Kassandra asks her father the boy's identity but he reacts angrily. Kassandra is sent to be fostered by Hecuba's sister Penthesilea, chief of the Amazons—a nomadic warrior tribe consisting only of women. There, Kassandra comes to love their lifestyle—though it is not without its trials—and she learns of her twin, continuing to experience visions of him. Kassandra sees the Judgement of Paris, in which her brother deems Aphrodite more beautifulthan Athena or Hera; Aphrodite rewards Paris by eventually promising the love of Helen of Sparta—daughter of Leda and Zeus.
In Colchis, ruled over by Queen Imandra, Kassandra undergoes the rites of a priestess and is told that serving the Goddess is her destiny. At the age of fifteen, Kassandra is unhappily returned to her home. She arrives during a festival in time to see Paris win and be revealed to his true parents as their son. Despite the prophecy, Priam and Hecuba happily welcome him home. However, Hector and his other brothers, jealous of the attention and achievements Paris has suddenly garnered, suggest that he be sent abroad to treaty with King Agamemnon—who holds Priam's sister. Paris readily agrees. Meanwhile, Kassandra begins training as a priestess of the temple of Apollo, despite misgivings that she is abandoning the Goddess. Part of her duties include helping care for the temple serpents—symbols of Python whom Apollo is said to have slain. Paris returns to Troy with the beautiful Helen, wife of King Menelaus, and she is welcomed into the city. Kassandra's warnings that Helen will destroy Troy go unheeded, and Paris denounces his sister as a madwoman.
At the temple, Kassandra is assaulted by Khryse, a priest who disguises himself as Apollo in order to seduce her. She sees through his trickery and fights him off, but the god feels insulted by her refusal and makes the city's residents stop believing her prophecies. Menelaus' brother Agamemnon uses Helen's flight as a pretext for war and soon begins launching daily raids on Troy, beginning the Trojan War. Kassandra spends more time with her family to help with daily tasks while the men—led by Hector—fight off the Akhaian invaders. The war continues sporadically despite the attempts of former Trojan ally Odysseus to end the conflict.
Two years into the war, Kassandra returns to Colchis to learn more of serpent lore. Along the way, she encounters Penthesilea. Kassandra is unhappy to find that the nomadic ways of life of the Amazon and Kentaur are ending, and that Penthesilea's tribe is dwindling in number. Kassandra experiences a horrifying vision of Apollo firing arrows indiscriminately at both armies—a sign of his wrath—which prompts her to return home. Accompanied with an adopted infant daughter named Honey whom she finds alongside the road, Kassandra returns to Troy and finds that the war is not going well for the Trojans.
Soon after her return, Apollo takes the form of Khryse and spreads a plague in the Akhaians' camp in response to Agamemnon's sacrilegious refusal to return Khryse's daughter, who has been Agamemnon's prisoner for three years. Khryse's daughter is reluctantly returned to her father and the Akhaian leader takes the young warrior Akhilles' concubine as reparation for his loss. The furious Akhilles refuses to continue fighting. Menelaus and Paris duel each other, but Paris flees the fight due to the intervention of Helen and Aphrodite.
Most of Kassandra's family has come to think of her as mad, and become angry when she feels compelled to vocalize prophecies that foretell the end of Troy. Despite Kassandra's warnings, the city experiences an earthquake sent by Poseidon, killing the three young sons of Helen and Paris. After Patroklus is killed by Hector, his closest friend Akhilles again joins the fight to get his revenge. Hector and his younger brother Troilus are killed, to the grief of everyone in Troy. Akhilles kills Penthesilea in battle, and soon after Kassandra fires a fatally poisoned arrow at his unprotected heel.
Poseidon sends another earthquake, knocking down Troy's defenses. The Akhaians flood into the city, and Kassandra and Honey are raped by the warrior Ajax. The women of Troy are divided up among the Akhaians, and Kassandra becomes Agamemnon's concubine. She is freed when his wife Klytemnestra murders him upon their return to Mykenae. Kassandra makes her way back to Asia Minor, where in the desert she hopes to recreate a kingdom of old—one ruled by a powerful queen.