The story follows the adventures of a fictional "Cyrus Spitama", an Achaemenid Persian diplomat of the 6th-5th century BCE who travels the known world comparing the political and religious beliefs of various empires, kingdoms and republics of the time.

Over the course of his life, he meets many influential philosophical figures of his time, including Zoroaster, Socrates, Anaxagoras, the Buddha, Mahavira, Lao Tsu, and Confucius.

Cyrus then goes into retirement, but is called upon by Xerxes' successor, Artaxerxes I, to serve as ambassador to Athens and witness to the secret peace treaty between Pericles and himself.

As noted in Vidal's own introduction, it can be considered a "crash course" in comparative religion, as during the story, the hero sits down with each of the religious/philosophical figures (apart from Socrates) and discusses their views.

Paul Theroux, in a contemporary review for The New York Times, took issue with Vidal's omission of "anything like plain vulgar domesticity", opining that its preoccupation with "the sort of patrician name-dropping that mars so many historical epics" made the book a good novel rather than "a great one".