The enchanter Merlin advises King Uther Pendragon to establish the fellowship of the Round Table, which will be comprised of the 140 greatest knights in the kingdom. Merlin is to continue his role of Uther’s counselor with Uther’s son, Arthur, who will maintain and immortalize the tradition of the Round Table.

Arthur’s life begins as the result of an illicit affair between Igraine, the duchess of Tintagel and the wife of Gorlois, and Uther Pendragon. Merlin’s magic art had allowed Uther to visit Igraine in the likeness of her husband, of whose death she is as yet unaware. Arthur is conceived as a result of this deception. Ignorant of his true origin, he is brought up from infancy by one of Uther’s knights.

In Arthur’s youth, the Lady of the Lake, Nimue, presents him with the sign of his kingship: Excalibur, a great sword encrusted with precious stones. Still ignorant of the identity of his mother, Arthur has a brief love affair with Morgause, the queen of Orkney and one of Igraine’s three daughters—and, thus, Arthur’s half sister. The product of this incestuous liaison is Mordred, who is both King Arthur’s nephew and his illegitimate son. Sir Gawain, a knight intensely loyal to Arthur, is the son of King Lot of Orkney and his queen, Morgause. Gawain is, therefore, Arthur’s nephew.

Arthur takes Guinevere as his queen. Lancelot, a French knight and warrior of almost superhuman capabilities, joins the Round Table and becomes the courtly lover of Queen Guinevere. He is practicing a medieval convention in which a knight chastely loves and honors a lady without regard to her marital status. This chaste love eventually becomes carnal, sowing the seeds of destruction for Arthur’s kingdom.

In Camelot, seat of Arthur’s court, Lancelot and Guinevere have begun a love affair. Mordred and Sir Agravain—one of Gawain’s several brothers, who dislikes Lancelot intensely—plot to capture Lancelot and the queen in flagrante. The king goes hunting, allowing Mordred and Agravain the opportunity to substantiate, if they can, their charges against the lovers. Lancelot indeed visits the queen’s chamber. The two conspirators and an additional twelve knights of the Round Table trap Lancelot within the queen’s chamber and demand that he surrender himself to them. When Lancelot finally emerges, he slays Agravain and his twelve companions. Only Mordred, wounded, escapes. Lancelot entreats Guinevere to go away with him but, grief-stricken at the disastrous results of her adultery, she tells him she will stay.

Guinevere is to be burned at the stake for her offense. Arthur bids Gawain and his brothers, Gaheris and Gareth, to lead the queen to the fire. Gawain respectfully declines, but his brothers reluctantly obey; they refuse, however, to bear arms. Lancelot rides to the queen’s rescue, slaying all who oppose him. Unfortunately, in the crush of battle, he unwittingly kills the unarmed Gaheris and Gareth. He takes Guinevere to Joyous Garde, his castle in England. Gawain, formerly Lancelot’s dear friend, now becomes his implacable enemy. After the pope arranges a truce between the forces of Lancelot and the king, Lancelot returns Guinevere. He and his kin leave England to become rulers of France.

Arthur, encouraged by Gawain, invades France and renews the war. Mordred takes advantage of Arthur’s absence and declares himself king. He attempts to marry Guinevere, but she escapes. Upon learning of Mordred’s treachery, Arthur and his army return to England. Gawain’s life is taken when a battle ensues on the landing grounds. Before he dies, however, he repents for pressing Arthur to make war on Lancelot. Arthur is urged in a dream to make a one-month truce with Mordred; the usurper agrees, but both he and the king tell their men to attack if a sword is brandished. Unluckily, a knight is bitten on the foot by an adder, and when he raises his sword to kill the serpent, a general battle breaks out. One hundred thousand participants are killed and, at the conclusion of the carnage, Arthur and Mordred meet in single combat. Arthur runs his son through with his spear but simultaneously receives a mortal wound to the head. Finally, only he and Sir Bedivere remain alive.

The dying Arthur instructs Bedivere to cast Excalibur into the lake. Bedivere, seduced by the richness of the sword, twice hides it and lies to the king. The third time, however, Bedivere obeys. A hand reaches up, grasps Excalibur, and draws it beneath the surface. Bedivere puts the king on a barge containing three queens clad in mourning—his sister, Queen Morgan la Fée; the queen of North Wales; and the queen of the Waste Lands, all accompanied by Nimue, the Lady of the Lake. As he is being rowed away, Arthur tells Bedivere he is going to Avilion (a possible earthly paradise) either to die or to recover from his wound. Bedivere later discovers a chapel where a hermit tells him of a number of ladies who had visited at midnight with a corpse for him to bury.

No one is certain that King Arthur is dead. The inscription on his tomb refers to him as the once and future king—he may come again if England needs him. Guinevere becomes a nun and Lancelot a priest, ever doing penance for their sins.