It is set in the Dark Ages, in the early years of the sixth century. The Romans left Britain a hundred years previously and now the Britons are fighting the invading Saxons. Unfortunately they are also fighting one another.
The story is told by Derfel, one of Arthur's warriors who has become a monk in his old age and is writing a chronicle of the events he has lived through. (There really was a warrior of this name who became a monk.) He is low-born—his mother was a Saxon slave—but his prowess in battle and his friendship with Arthur lead to his becoming a lord. Derfel is devoted to Arthur although he is not blind to his hero's faults, notably his passion for Guinevere which almost precipitates a total disaster. But at the end of the book Arthur has succeeded in unifying the warring kingdoms under his leadership.
As well as Arthur and Guinevere we meet other well-known figures from the Arthurian legend, including King Uther, Mordred, Merlin, Lancelot, and Galahad. Their characters mostly differ from the prototypes we are familiar with, especially Lancelot, who appears as a vain intriguer who presents himself as a valorous warrior while avoiding any actual fighting. Merlin is a Druid whose goal is to restore the ancient religion of Britain which was largely destroyed by the Romans.
Most people in the story, including Derfel at this stage, are pagan, although there is an increasing number of Christians. These mostly appear in an unfavourable light, apart from Bishop Bedwin, who is a warrior and a Mithraic initiate. Everyone, except perhaps Arthur, is deeply superstitious, terrified by omens and spells. At least two of the kingdoms, Dumnonia and Powys, really existed; the status of others is uncertain. A map is provided but in the Kindle version the names are mostly too small to read.
In this book Arthur is a warlord but not a king; he is Uther's illegitimate son. He has sworn to protect Mordred, Uther's grandson and designated heir, and to ensure the survival of his kingdom Dumnonia (roughly Devon and parts of Somerset and Cornwall). The story is full of vividly described violence of all kinds—murder, rape, and of course battles. As Cornwell remarks, sixth-century Britain must have been a horrid place. But it makes a good story.