Unlike the previous installments in this series, this one is recounted by Aelric in his extreme old age, as he approaches the century mark. Decades after Aelric preserved Egypt (and its grain shipments) for the Empire, and after the armies of the Caliphate rendered it all wasted effort, he spends his final years in a Kentish monastery teaching classical languages to the children of Anglo-Saxon peasants.
...Until he is sucked back once again into the politics of clashing empires.
The Umayyad Caliph Muawiya, having defeated the partisans of Ali in a civil war, reigns from the recently conquered city of Damascus. The Caliphate conceals barely suppressed political divisions, as the family and close associates of the Prophet simmer with resentment back in the Hejaz, and Muawiya creates a new aristocracy of his own from converted Greeks and Syrians. Of course the Byzantine Empire, shrunk to a core of the City and its European provinces, has its spoon in the political stew of the Caliphate. And all these factions, each for reasons of its own, want Aelric -- who once saved the City with his invention of Greek Fire -- in Damascus.
The first-person perspective, from Aelric's extreme old age, of the transitoriness of human affairs, and overshadowed by the prospect of his mortality, give this story bittersweet overtones lacking from the others.