In Book One, we learn how Viviane, the Lady of the Lake, uses her magical powers to ensure that an acceptable King will reign in Britain: a king, that is, who will be mindful of the ways of Avalon as well as of Christianity. So Viviane has her sister Igraine and Uther Pendragon, a well-known knight, fall in love and marry. Uther is chosen by a counsel of knights to become the next king after King Ambroise's death, and Igraine is a priestess of Avalon. They have a son, Arthur, who will be the great High King. As a result of his parents and their faiths, he will be sympathetic to both Avalon and the Christian world.

The reader not only learns of Arthur's birth and the mystical guidance, but of the incestuous relationship between Arthur and his half-sister, Morgaine. The two siblings take part in a ritualistic event, where Arthur takes the guise of The Stag. However, after their night together, Morgaine realizes that it was Arthur with whom she slept the previous night. Ashamed and angered, Morgaine leaves Avalon and ends her apprenticeship with Viviane because Viviane was responsible for her night with Arthur. The book ends with Morgaine going to her Aunt Morguese because Morgaine is pregnant with Arthur's child.