Mordred makes himself King of England and incestuously claims Guinevere as his wife. Guinevere escapes to the Tower of London. The Bishop of Canterbury reproaches Mordred for his usurpation and would-be incest, and when Mordred tries to kill him, he flees and becomes a hermit. Mordred wins many Englishmen to his side, then meets Arthur at Dover but is forced to retreat from him.
In this battle Gawain is mortally wounded. As he dies he admits to Arthur that if it were not for his insane pride in insisting on unjust revenge, Launcelot would be here now to save the kingdom; then he writes Launcelot, begging him to come help Arthur and also to pray at his tomb. Then, bleeding from the wound he got originally from Launcelot — with the fated sword of Balyn — Gawain dies.
Arthur meets Mordred again at the battle of Bareon Down and again puts him to flight. They meet next at Salisbury Plain, and there, with all who loved Launcelot fighting on Mordred's side, they prepare for what is to be their last battle. The night before the battle, Arthur dreams he is on the Wheel of Fortune, sitting on a throne and dressed in the richest gold that can be made:
And the kynge thought there was undir hym, farre from hym, an hydeous depe blak watir, and therein was all maner of serpentis and wormes and wylde bestis fowle and orryble. And suddeynly the kyrige thought that the whyle turned upso-downe, and he felle anionge the serpentis, and every beste toke hym by it lynnue. And than the kynge cryed as he lay in hys bed, "Helpe! Helpe!"
After the prophetic dream he has another. Gawain and a number of ladies come to him to warn him against fighting in the morning for if Arthur fights, he will die; if he waits for a month, Launcelot will be here to help him. Then Gawain and the ladies vanish.
Arthur asks a truce, and the two armies meet on the field to set terms. An adder appears, a knight unthinkingly draws his sword to kill it, and the two armies are at war. At the end of the day, Mordred is the only man of his army left standing, and Arthur has only two knights, Sir Lucan and Sir Bedivere. Against Sir Lucan's advice, Arthur fights Mordred and kills him, but he gets his own death wound as he does it. Lucan and Bedivere bear him to a chapel. Robbers overrun the battlefield stealing the gear of dead knights, killing any that have life left in them.
Arthur is dying and cannot be moved to safety. And so he sends Bedivere to throw Excalibur into the lake nearby, then return and tell what he has seen. Bedivere hides the sword under a tree, thinking it too precious to throw away, then returns and says he has obeyed. "What did you see?" Arthur asks. Bedivere says he saw only waves and winds. Arthur sends him twice more, and the last time Bedivere does as he has been commanded. A hand catches the sword and brandishes it three times.
Then at Arthur's command, Bedivere carries the king to the waterside, where a barge awaits him and some ladies in black hoods. Bedivere puts Arthur in the barge and he is borne away to Avilon, perhaps to heal his wounds, perhaps to die. Bedivere wanders through a forest until he comes to where a hermit is kneeling over a fresh grave. It is the grave of a man brought to him at midnight by ladies in black. Whether or not the body is really that of Arthur, no one knows. Some say Arthur still lives, and some say riot.