Set in both Arthurian and post-Arthurian Britain, this novel is a little confusing at first, but stick with it.
Once you grasp that the hermit Budoc, protagonist of the post-Arthurian story, is one with the knight mab Petroc, hero of the Arthurian story ("mab" being the ancient British form of the Scottish "mac", son of) and that Budoc is the last of those once known as the Companions of Arthur, it all comes clear. Though it is not until the end of the book that we learn who exactly Budoc / map Petroc was. (To name him would be a definite spoiler.)
Budoc the hermit lives on a hill above a fishing village on the south coast of Cornwall, in ancient Dumnonia. Life is peaceful, for him and for the villagers. But then, suddenly, trouble comes out of the blue, and troubles do not come singly. A band of Scotti (Irishmen) arrive in the village in search of a sacred chalice which, or so they have been told, the local hermit has in his possession. They proceed to massacre the inhabitants of the village. The three-page description of the massacre is detailed and horrifying, but also, because we see it through the eyes of a village girl, very moving. Then a boat full of Saxons anchors off the beach. They are looking for somewhere to settle and know nothing of the massacre or the presence there of the Irish warriors. The hermit, and the local girl, who survived the slaughter, hide in the forest.
Meanwhile, back in Arthur's time, the big question is whether, following his overwhelming victory against the Saxons at the Battle of Badon, Arthur will be content to remain the Magister Militum, Commander of the Armies of Britain, or will declare himself Emperor. It transpires that to do so legitimately, he must travel north to Iardomnan (the Hebrides) and pass a test and receive a chalice from the priest of the Attecotti, the first-comers. And mab Petroc, our hero, has fallen in love with a mysterious female bard who sets out with them on the long journey on horseback to the north coast of Dumnonia and by ship up through the Irish Sea to the Western Isles of Scotland.
Don't miss McCormack's ten-page appendix on the historical background to the novel. The story is full of myth and magic, and I wouldn't have it any other way, for so was Britain at the time, but it is not fantasy in any sense; it is realistic and historically accurate. All the author has done in effect is to give the name Arthur to the Commander of the British forces at Badon and go from there.