The novel is divided into five parts narrated by the protagonist, Derfel Cadarn. A former warrior sworn to Arthur and now an elderly monk, Derfel tells the story of how Arthur became a warlord in Dark Age Britain despite illegitimacy to the throne. After being banished to Armorica by his father, the High King Uther Pendragon, Arthur returns to protect the new king, his infant nephew Mordred. However, on the cusp of a united peaceful Britain, Arthur's decision to marry Guinevere plunges the island into war again.