Arthur's nephew, the villainous Modred, has been planning for a long time to usurp the throne. He is assisted in his evil designs by Vivien. Together, they take advantage of every opportunity to arouse discord and treason at the court.
One night, while the king is away, Modred is able to trap Lancelot and the queen in her chamber. In the confusion that follows, several of Modred's followers are slain, Lancelot flees to his feudal domain in France, and Guinevere takes refuge in the abbey at Almesbury. Here she is given sanctuary by the nuns even though they are not aware of her real identity.
For the next few weeks, Guinevere lives at the abbey, suffers from a serious depression, and speaks with no one except the young novice who serves as her maid. One night they receive startling news: Arthur, who believed Lancelot a traitor, has been waging war on him in France. Meanwhile, he left Modred as regent in his place. After having formed an alliance with the northern heathen and various unfaithful lords, Arthur's wicked nephew has made himself king. Arthur is now returning to England with his army.
When Guinevere learns this development and realizes that the awful state of the kingdom is in large part due to her own behavior, she moans: "With what a hate the people and King must hate me."
The young novice attempts to cheer the weeping lady but has little success. In order to distract her, the nun repeats all the old stories and prophecies about Arthur, the great achievements of his reign, and the eventual decay of his Round Table. She attributes the moral downfall to the sin first committed by the queen and Lancelot. Upon hearing this, Guinevere's grief becomes more intense. She orders the nun to leave her chamber.
Alone, Guinevere muses about herself and remembers some happy episodes of her life with Arthur. Her thoughts ramble on and she indulges in self-pity. Suddenly an armed knight rides into the courtyard, and a whisper runs through the abbey: "The king! The king!" A few seconds more and Arthur confronts Guinevere in her room.
The king's demeanor is saddened, for he has at last learned the truth about Guinevere's infidelity, and now he foresees his own impending defeat and death. He is, however, a majestic figure as he stands before her. Arthur speaks to his wife at great length, saying in part:
". . . I did not come to curse thee, Guinevere. . . .Lo, I forgive thee . . . do thou for thine own soul the rest. . . .Let no man dream but that I love thee still. . . .Hereafter in that world where all are pure
We two may meet before high God, and thou
Wilt spring to me, and claim me thine, and know
I am thine husband. . . . Leave me that,
I charge thee, my last hope. Now must I hence.. . . to lead mine hosts
Far down to that great battle in the west,
Where I must strike against the man they call
My sister's son . . . who leagues
With . . . heathen, and knights,
Traitors — and strike him dead, and meet myself
Death, or I know not what mysterious doom.
And thou remaining here wilt learn the event;
But hither shall I never come again,
Never lie by thy side, see thee no more — Farewell!"
After Arthur leaves, Guinevere becomes hysterical for she has realized that she still loves him and understands at last the full significance and consequence of her immorality.
In the years that follow, she remains at the abbey and devotes her life to penance and good works. After a while, in virtue of her good deeds and pure life, she is made abbess. She dies there, beloved by the nuns and all the inhabitants of the surrounding country.