The novel is set in the fifth century B.C. and spans the ancient world from Persia to India to Cathay (ancient China). The blind, 75-year-old narrator, Cyrus Spitama, Persian ambassador to Athens and grandson of the prophet Zoroaster, begins dictating his memoirs in December 445 B.C. to his 18-year-old nephew Democritus. Prompted by outrage at a public reading by the historian Herodotus, whose account of the Persian Wars Cyrus considers slanderous, he resolves to set down the true history of his era, including his travels across the known world. His account moves between the present-day frame in Athens, where he hosts dinner parties with figures like the sophist Anaxagoras and Elpinice, the formidable sister of the late general Cimon, and the vast sweep of his life story.
Cyrus's earliest memory is the murder of his grandfather Zoroaster at a fire altar in the frontier city of Bactra. Seven-year-old Cyrus stands beside the prophet during a sacred ceremony when Turanian raiders attack, slaughtering the priests. Zoroaster continues the ritual even as he is struck down, speaking the words of the Wise Lord (Ahura Mazdah) with his dying breath. A surviving priest rescues the boy. Hystaspes, the satrap (provincial governor) and father of the future Great King Darius, escorts Cyrus and his Greek mother Lais to the imperial court at Susa, charging the boy with continuing Zoroaster's mission.
At court, young Cyrus navigates dangerous factions. His mother Lais, secretly a Thracian witch, cultivates the friendship of Queen Atossa, daughter of Cyrus the Great, who schemes to secure the succession for her son Xerxes over the elder prince Artobazanes. Cyrus earns a fearsome reputation among the Magian priests, the hereditary priestly caste, by publicly chanting Zoroastrian hymns against their practices. Atossa, initially hostile, transfers Cyrus to the school of the royal princes, where he becomes the inseparable companion of Xerxes.
As the boys grow into young men, Persian politics turn westward. At a secret council, the exiled Athenian tyrant Hippias petitions Darius for military support, and the Great King authorizes a fleet to conquer the Greek island of Naxos, setting in motion the Persian Wars. Cyrus accompanies his friend Mardonius on campaigns to Sardis, the capital of Lydia in Asia Minor, where Greek rebels burn the city, providing Persia's pretext for war against Athens.
Darius, however, privately reveals that his true ambition lies eastward. He commissions Cyrus as ambassador to the Indian kingdoms, ordering him to survey the subcontinent's wealth, military resources, and religions in preparation for future conquest. Cyrus sails from the Persian Gulf to the Indus delta and travels overland across the Gangetic plain, entering a world of bewildering religious diversity.
In India, Cyrus encounters thinkers whose ideas challenge his Zoroastrian certainties. Gosala, a grim ascetic, teaches absolute determinism: every soul must undergo 84,000 rebirths, and no action can alter this course. He throws a ball of string into the air and lets it unwind, an image that haunts Cyrus permanently, and dismisses Zoroaster's concern with religious procedure as a sign of "extreme youth." Mahavira, leader of the Jains, a sect devoted to nonviolence and the liberation of individual soul-units through cycles of rebirth, speaks broadly of "youthful religions" falling short of absolute truth. At a Vedic fire temple, Brahman priests chant a creation hymn questioning whether even the highest god knows how the world began.
Cyrus arrives at the court of King Bimbisara of Magadha and marries Ambalika, the 12-year-old daughter of Bimbisara's ambitious heir, Prince Ajatashatru. Prince Jeta, a Koshalan nobleman and devoted Buddhist, introduces Cyrus to the Buddha's four noble truths: that life is suffering, that desire causes suffering, that suffering can cease, and that the eightfold path leads to cessation. Cyrus later meets the Buddha himself, a small, golden-skinned old man who refuses to adopt any theory about the world's origin, comparing metaphysical questions to a wounded man demanding to know his archer's identity before allowing a poisoned arrow to be removed.
While Cyrus is in India, Ajatashatru murders his father Bimbisara and seizes the throne. In neighboring Koshala, Prince Virudhaka similarly overthrows and kills his own father. Cyrus departs with a westbound caravan, deeply disturbed by these parricides but carrying a transformed understanding of the world's religions.
Back in Persia, the expedition against Athens has failed at Marathon. Darius, aging and partially paralyzed, collapses from his throne. Egypt rebels, and Darius dies. Xerxes becomes Great King after averting a challenge from his brother Ariamenes. In a moment of dark intimacy, Xerxes reveals the dynasty's central secret: Darius was not the legitimate heir but murdered both sons of Cyrus the Great, inventing the story of a Magian usurper to justify his coup. Xerxes believes he is cursed by his father's crimes.
Cyrus is sent as ambassador to Cathay, crossing a vast desert that kills most of his caravan. Captured by the state of Ch'in, the westernmost Cathayan duchy, he is held for six months by the ruthless prime minister Huan before escaping with the help of the eccentric duke of Sheh. At Loyang, the shadow capital of the Middle Kingdom, Cyrus meets Master Li (Lao Tzu), keeper of the Chou archives, who expounds the doctrine of the Tao, or Way: a primal unity preceding all opposites, where the ideal is
wu-wei, doing nothing that is not natural or spontaneous.
After passing through several Cathayan states, Cyrus reaches the duchy of Lu, where he recognizes his Cathayan friend Fan Ch'ih on a battlefield. Baron K'ang, Lu's hereditary dictator, secures Cyrus's release and commissions him to spy on the returning sage Confucius. Over months of fishing conversations by the river, Cyrus discovers in Confucius the wisest man he has ever known. Confucius articulates his golden rule: "Never do to others what you would not like them to do to you." He insists that goodness comes from the ruler's personal example, not from laws and punishments. When Cyrus asks about death, Confucius replies, "Since you do not yet understand life, how can you understand death?" In their final meeting, the old sage admits he has fallen short of his own ideals: "I am not what I would want to be." Cyrus, despite disagreeing with Confucius's atheism, finds him "altogether good."
Returning to India years later, Cyrus finds the Gangetic plain devastated by Ajatashatru's conquests. He reunites with Ambalika and meets his two sons at Shravasti, but Ambalika arranges a forged death certificate so she can remarry, insisting the boys remain in India. Cyrus slips away disguised as a traveler, never to see his Indian family again.
The final sections trace Xerxes' decline. After launching his invasion of Greece, where Mardonius is killed at Plataea and the fleet defeated at Mycale, Xerxes retreats into the harem and his building projects, obsessed with women he cannot possess. His seduction of his own son's wife provokes Queen Amestris to mutilate the mother of the girl, driving Xerxes' half-brother Masistes into fatal rebellion. Xerxes is murdered in his sleep by his guards commander and court chamberlain, with the queen's tacit consent. Crown Prince Darius is framed and executed, and the 18-year-old Artaxerxes, Amestris's favored second son, takes the throne.
Shortly after dictating these events, Cyrus confesses to Democritus that "something is missing" from every system of belief he has encountered. His lifelong quest to understand creation remains unresolved. Democritus appends his own philosophy, declaring that the first principles of the universe are "atoms and empty space."
Under Artaxerxes, Cyrus translates for the fugitive Athenian general Themistocles, who has fled to Persia after being ostracized. After a secret peace is negotiated, Cyrus is dispatched to Athens as the living symbol of the treaty. Cyrus dies peacefully a week after a reception at the house of Aspasia, Pericles' Milesian companion, while listening to a reading from Herodotus. In an epilogue written decades later, Democritus notes that Pericles died during the plague and Athens surrendered to Sparta after 28 years of war. He dedicates the work to "the last living survivor of a brilliant time, Aspasia, the wife of Lysicles, the sheep-dealer."