The book centers around Byzantine Emperor Justinian II. It is presented as the emperor's autobiography, as it is being read to one of his faithful aides, the blind soldier Myakes. The book follows Justinian's time before and after taking the throne, as well as his overthrow, mutilation and exile in the Crimea, his subsequent return to power (following a possibly apocryphal nose-job), his insane quests for revenge, and finally being unseated a second time and executed.
Harry Turtledove, also known by his pseudonym "H.N. Turteltaub", has a doctorate in Byzantine history, and most of what's in the book is historically accurate. The parts that are pure conjecture, such as certain names and the way Justinian's mutilation was taken care of, are mentioned as being conjecture in the Author's Note.
The central theme of the book appears to be "power corrupts, and absolute power corrupts absolutely", but this is not necessarily the case. Justinian is a man who is shown to believe that he can do no wrong. His mindset seems to be that he's on the throne because God wants him to be there. That mentality answers the question, "If I am chosen by God, how can I do evil?".
Another central theme of the book is the importance of religion in the 700s. Justinian's father is shown convening a synod, and both Justinian and his father lead battles against the newly arisen Muslim faith. The Popes, who are considered by many in Constantinople to be merely the Bishop of Rome, are shown as not having as much influence then as they would have in later centuries.