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 of the time.
Cyrus, who is the grandson of Zoroaster and who survives his murder, grows up at the Achaemenid court as a quasi-noble, and becomes a close friend of his schoolmate Xerxes.