The Mists of Avalon is an epic tale of love, loyalty, betrayal, kingship, and magic. It takes place over several decades and two generations. It tells the fabled tale of Camelot and the Knights of the Round Table. Taking a different view from the legend, it is told from the perspective of the women of Avalon. These powerful women use their magic and their wiles to fulfill the will of the Goddess and place a king on the throne of Britain, as well as later take him down from it.

Every event in the story is laced with some element of magic. As the Lady of the Lake seeks to place Uther Pendragon on the throne as the High King, she upsets the lives of many. Those such as Igraine, who are living quiet lives, are thrown into the fray as part of the master plan. Igraine gives birth to Arthur, who eventually becomes the High King when Uther dies. Before Arthur can become king, he needs to be tested by trials that Avalon puts him through. As part of these trials, he makes a marriage with the land and has a son from the union which he does not learn of for many years.

Arthur swears an oath to Avalon to always defend it and its old ways and rituals. In turn, Viviane and the Merlin place Arthur on the throne and give him his charmed sword, Excalibur, as well as the Holy Regalia of Avalon. While he swears this oath to Avalon, it will, in turn, continue to support him in fighting for peace for the land.

Shortly after Arthur becomes king, he takes Gwenhwyfar as his bride. There is trouble from the beginning as it is apparent that she and Lancelet are in love with each other. Still, Gwenhwyfar and Lacelot do as they must and live apart from each other in agony. Years later, when Gwenhwyfar fails to produce an heir for Arthur, Lancelet's son Galahad is named heir to the throne. It is also in later years that Arthur gives in to Gwenhwyfar's demands to put aside the old ways of Avalon for the newer, Christian ways. This embracing of Christian ways is at the heart of the action, because Avalon now seeks to take Excalibur and the Holy Regalia back. If Arthur will not support them, they will no longer support him.

Years later, Arthur's son, Gwydion, grows up and becomes powerful in the Sight as well as the ways of Avalon. As the prophecy decrees, Gwydion challenge Arthur for the throne, once he discovers Gwenhwyfar and Lancelet's affair. It splits the Companions, and Gwydion's challenge for the throne is the undoing of Camelot. The quest for the Grail and the return of Excalibur lead to several deaths and the end of an era.