Britain is a land divided into small Celtic kingdoms in the process of being conquered by the more united Saxon invaders. When Uther, the Pendragon or High King, dies without legitimate sons, any semblance of a unified defense vanishes. Only Arthur, Uther's son, continues to fight the Saxons, but as a bastard, he can only rely on the support of his late father's warband and the kingdom of Dumnonia. A civil war is in the offing as the rest of the underkings plot to claim the vacant throne.
One of the most powerful of the schemers is Lot, King of the Orcades in the far north. He has three sons by his wife, Morgawse, Uther's legitimate daughter and a notorious witch. Agravain, the eldest, is a straightforward, gifted warrior. The second, Gwalchmai, is clever, but a poor fighter, favored by his mother. Finally, there is Medraut, who resembles Lot so little that many question his parentage.
Lot and Agravain go off to fight in Britain. Gwalchmai despairs of becoming a warrior and asks his mother to teach him witchcraft instead. Medraut, who looks up to his brother, wants to learn magic as well, but Gwalchmai dissuades him.
When the Saxon King Cerdic of Wessex invades Dumnonia, Arthur realizes that the only way to protect Britain is to end the civil war. He therefore proclaims himself the Pendragon. A brilliant general, Arthur defeats several kings, one after the other. The remaining contenders then unite against him, but Arthur wins a decisive battle and forces them (including Lot) to swear the Threefold Oath of Allegiance to him.
Morgawse is furious and prepares black magic to strike down her half-brother Arthur, with Gwalchmai's help. To his dismay, he finds Medraut a willing participant. When he learns that a human sacrifice is required, Gwalchmai kills the bound victim to spare him an agonizing death and flees.
An otherworldly boat appears and transports him to the Land of the Blessed, where he meets his kinsman, the god Lugh of the Long Hand. Gwalchmai pledges his allegiance to the Light and is given a magical sword. He is then returned to Britain to fight for Arthur.
However, Gwalchmai stumbles upon a band of Saxons and is made Cerdic's thrall. King Aldwulf of Bernicia, a sorcerer and ally of Cerdic, has captured, but cannot control a supernatural horse. Gwalchmai tames it and rides away. He names it Ceincaled.
Gwalchmai tries to join Arthur's warband, but the suspicious High King refuses his service. Arthur has heard rumors that Gwalchmai is a sorcerer. Nonetheless, he cannot turn away his own nephew. Gwalchmai fights loyally for Arthur, earning a reputation as the finest cavalry fighter in Britain. He makes friends, among them Bedwyr, Arthur's most trusted advisor, but the High King remains distrustful and the warband is strongly divided regarding him.
When Gwalchmai is wounded in battle, he recovers in a friendly holding under the care of Gwynhwyfar, the daughter of the clan leader. When he is well enough, he leaves to rejoin Arthur. At the outskirts of Arthur's camp, he tries to save a peasant woman's badly wounded husband, but the man dies.
Gwalchmai meets Arthur. He surprises everyone by announcing that, because he has divided the warband, he is leaving. When the peasant woman shows up to thank Gwalchmai for his efforts, Arthur is finally convinced that he has been wrong. He asks Gwalchmai to stay.