The Mists of Avalon is a feminist reinterpretation of Arthurian legend centered on the character of Morgaine, a sorceress tasked with protecting the Pagan traditions of Old England. She is set against King Arthur and his Christian court and wrestles with her role and with her love for Lancelet, one of Arthur's knights. In the end she is able to make peace with the emergence of Christianity, realizing that it is only another iteration of the divine Mother Goddess she worships.
The Mists of Avalon is a generations-spanning retelling of the Arthurian legend that brings it back to its Brythonic Celtic roots (see Matter of Britain). The plot tells the story of the women who influence King Arthur, High King of Britain, and those around him.
The book's main protagonist is Morgaine, priestess of Avalon, who is King Arthur's half-sister. Their mother, Igraine, is married to Uther Pendragon after Morgaine's biological father, Gorlois, is killed in battle. Rumors spread in Avalon that before Igraine knew her husband Gorlois was killed, Uther consulted with Merlin who used his magic to transform the king into the likeness of Gorlois and thus gain access to Igraine at Tintagel. He spent the night with her and they conceived a son, Arthur. Morgaine witnesses Uther Pendragon's accession to the throne of Caerleon after his predecessor, Ambrosius, dies of old age. Uther becomes her step-father, and he and Igraine have a son, Arthur, Morgaine's half-brother.
When Morgaine is eleven years old and Arthur six, an attempt of murder is made on Arthur's life. Their maternal aunt, the high priestess Viviane, arrives in Caerleon and advises Uther to have Arthur fostered far away from the court for his own safety. Uther agrees, and also allows Viviane to take Morgaine to Avalon, where she is trained as a priestess of the Mother Goddess. During this period, Morgaine becomes aware of the rising tension between the old Pagan and the new Christian religions. After seven years of training, Morgaine is initiated as a priestess of the Mother, and Viviane begins grooming her as the next Lady of Avalon.
Some months after her initiation, Morgaine is given in a fertility rite to the future high king of Britain. Their union is not meant to be personal, but rather a symbolic wedding between the future high king and the land he is to defend. The following morning, Morgaine and Arthur recognize each other and are horrified to realize what they have done. Two months later, Morgaine is devastated to find that she is pregnant.
After Uther dies in battle against the Saxon invaders, Arthur claims the throne of Britain despite questions about his legitimacy (he had been conceived within days of Igraine's marriage to Uther Pendragon). Since Arthur must now defend Britain against the Saxons, Viviane has Morgaine make him an enchanted scabbard that will prevent him from losing blood and gives him the sacred sword Excalibur. With the combined force of Avalon and Caerleon, Arthur repels the invaders.
As Morgaine's unborn child grows within her, so do her feelings of anger and betrayal toward Viviane, who she believes tricked her into bearing a child to her own half-brother. Unable to stay in Avalon any longer, she leaves for the court of her aunt Morgause, queen of Lothian, where she bears her son, naming him Gwydion. Spurred by her husband Lot's ambition and her own, Morgause tricks Morgaine into allowing her to rear her son. To escape Lot's unwanted advances, Morgaine leaves Lothian and returns to Arthur's court as a lady-in-waiting to the high queen, Gwenhwyfar. She does not see Gwydion again until he is grown and a Druid priest.
When Gwenhwyfar fails to produce an heir, she is convinced God is punishing her for her sins. Chiefest among them, as she believes, are her failure to persuade Arthur to outlaw pagan religious practice in Britain and her forbidden love for Galahad, Arthur's cousin and finest knight, who is also known as Lancelet. Although Lancelet reciprocates Gwenhwyfar's love, he is also Arthur's friend and an honorable man. This situation causes terrible suffering to both Lancelet and Gwenhwyfar.
On the eve of a decisive battle against the Saxons, Gwenhwyfar prevails upon Arthur to put aside his father's Pendragon banner and replace it with her own Christian banner. As her religious fanaticism grows, relations between Avalon and Camelot grow strained. Still, in her desperation over her failure to carry a child to term, she asks Morgaine for help, threatening to have an extramarital affair so she can become pregnant. In an attempt to keep Gwenhwyfar from doing so, Morgaine reveals that Arthur already has a son, though he does not know it.
After the battle, Arthur moves his court to Camelot, which is more easily defended than Caerleon had been. Seeking to free both Lancelet and Gwenhwyfar from the forbidden love that traps them both, Morgaine tricks Lancelet into marrying Gwenhwyfar's cousin, Elaine. Some time later, during a heated argument with Arthur over their lack of an heir, Gwenhwyfar breaks Morgaine's confidence and tells Arthur he has a son. In growing suspicion and horror, Arthur summons Morgaine and orders her to tell him the truth. Morgaine obeys. Now believing that the lack of a royal heir is God's punishment for Arthur's union, however unwitting, with his own half-sister, Gwenhwyfar urges Arthur to confess the encounter to the bishop, who imposes strict penance upon him. Then she and Arthur arrange for Morgaine to marry into Wales, far from Camelot. But because of a misunderstanding, Morgaine, who had thought she would be marrying the king's younger son Accolon, a Druid priest and warrior, finds herself betrothed to King Uriens of Wales, who is old enough to be her grandfather. Arthur yearns to meet his son Gwydion and perhaps foster him at Camelot, but each time he brings up the subject with Gwenhwyfar, she refuses to discuss it.
Morgaine marries Uriens and moves to Wales, but in time begins an affair with Accolon. The "old people" of the hills, who keep to the old pagan ways, regard Accolon and Morgaine as their king and queen. King Uriens suspects nothing, but Accolon's older brother, Avalloch, begins to; at one point, he confronts Morgaine in private and tries to blackmail her into sleeping with him as well. Morgaine sends Avalloch out on a boar hunt and is magically present when the boar kills him. In his grief for his eldest son and heir, Uriens abstains from pork for the rest of his life. Morgaine tells Accolon, who is now Uriens's heir, of the sacred marriage she made with Arthur years before. She adds that they must take the kingdom back from Arthur and the Christians and bring it back under the sway of Avalon. The attempted coup fails and Arthur kills Accolon in single combat. As Uriens recovers from the shock of losing a second son, Morgaine leaves Wales forever.
Gwydion, now grown, goes to the Saxon courts to learn warfare far from Arthur's eye. Impressed by his cleverness, the Saxons name him Mordred ("Evil Counsel"). Years later, at a Pentecost feast at Camelot, he introduces himself as Queen Morgaine's son and Queen Morgause's foster son, though he calls Queen Morgause "Mother" and Morgaine by her name. Because of his close resemblance to Lancelet, he must often tell people that Lancelet is not his father. To earn his knighthood with no suspicion of preferential treatment, Gwydion challenges Lancelet to single combat during a tourney and they fight. As they start to fight in earnest, Gwenhwyfar, who has warmed to Gwydion in the meantime, protests and Arthur interrupts the match. Lancelet makes Gwydion a knight of the Round Table, naming him Mordred.
When the knights of the Round Table leave to search for the Holy Grail, Mordred attempts to usurp the throne. In a climactic battle, the armies of Arthur and Mordred fight and Arthur is mortally wounded. Morgaine takes the dying Arthur through the mists to Avalon, reassuring him that he did not fail in his attempt to save Britain from the approaching dark times. Arthur dies in her arms as the shoreline comes into view. Morgaine buries him in Avalon and remains there to tell the tale of Camelot.