Lancelot and Guenever have a few tender moments before Guenever starts to fear that someone will discover them. Just as she is pushing Lancelot toward the door, they see that someone is trying to free the lock. Mordred, Agravaine, and twelve other fully armed knights are waiting outside the door. Lancelot manages to kill Agravaine and steal his sword and armor. He exchanges rings with Guenever and steps outside to face his enemies.
A week later, Gareth, Gawaine, Gaheris, and Mordred are sitting in the Justice Room. Mordred is criticized for running away from Lancelot, who slew all of the other knights outside of Guenever’s chamber. Lancelot has since escaped back to his castle. Guenever has been convicted of adultery and is to be burned at the stake. Everyone expects Lancelot to rescue her. Arthur, at Mordred’s urging, asks Gareth and Gaheris to join the guards who are already posted around her. They go unwillingly, without their armor. Just as Guenever is about to be burned, Lancelot and his knights arrive and rescue her. Arthur and Gawaine are both elated, and Arthur begins to plan out how their parties can be reconciled. Mordred returns from the scene and tells them that Lancelot killed Gareth and Gaheris, even though they were both unarmed. Neither Arthur nor Gawaine believes Mordred at first. Gawaine goes to confirm the news and returns heartbroken after finding Gareth and Gaheris dead.
Six months later, Lancelot and Guenever are in Lancelot’s castle, called Joyous Gard, which is under siege. Lancelot tells Guenever that he does not remember killing Gareth and Gaheris, but that he may well have done so in the confusion. Guenever decides that the only way to save themselves, and Arthur, is to ask the Church to intervene.
The Church agrees to mediate a peace. Back in Camelot, the bishop of Rochester presides over the decision. Lancelot is banished from England, and Guenever returns to Arthur. Gawaine swears, however, that he will still try to get his revenge on Lancelot.
Arthur and Gawaine follow Lancelot to France and place one of his castles under siege. Back in England, Guenever and her maid Agnes are knitting together in her chambers when Mordred enters. He is now quite mad, excessively dandy, and consumed by cruelty. He has founded a new political order, called the Thrashers, who speak of the old Gaelic wars and want to massacre Jews. Mordred tells Guenever that he plans to announce that Arthur and Gawaine have been killed in France and that he is the new king of England. Mordred also tells Guenever that he intends to take her as his wife.
In Arthur’s tent on the battlefield in France, Gawaine is trying to recover from a blow he receives during a fight with Lancelot. It is the second time Lancelot has defeated Gawaine and the second time Lancelot has chosen not to kill him. The bishop of Rochester enters with a letter from Guenever. It tells Arthur about Mordred’s plot. Arthur decides to lift the siege and return to England immediately. Gawaine, who is badly injured, insists on going as well, anxious to wreak revenge on his treacherous brother.
In his castle, Lancelot has just received a letter from Gawaine, who is now in England, telling him what Mordred has done. Gawaine writes that Guenever and her allies are defending themselves in the Tower of London, which Mordred is attacking with cannons. Gawaine forgives Lancelot for killing Gareth and Gaheris and asks him to return to England to help Arthur. He adds that his wound from Lancelot has reopened in battle and that he has been “hurt to the death.” Lancelot and his comrades decide to return to England at once.
Arthur is in his pavilion on the battlefield at Kent, where his forces are fighting Mordred’s army. It is late at night and he is working on his laws. Arthur begins to think about the reasons for war and what can be done to stop it from occurring. He calls for a page named Thomas Malory and tells him that Malory must remember the story of King Arthur, particularly the idea that might must be used for right. Arthur asks Malory to spread Arthur’s message of justice and peace. Malory agrees to do so, and after Malory leaves, Arthur begins to cry. He thinks of the lessons he learned from the animals when he was a young boy. He wakes up with a fresh mind, hopeful for the day when his dream will be fulfilled. He stands up and prepares himself for his final battle.
In this section, the delicate relationships between Arthur, Guenever, Lancelot, and the Orkney faction quickly unravel. The first step in the final destruction of Camelot is Mordred and Agravaine’s discovery of Lancelot in Guenever’s room. Although Lancelot saves himself from being killed, he has promised Arthur that he will not kill Mordred, and he is forced to leave one witness alive. Mordred then accuses Guenever of adultery. Arthur must follow his own laws and honor the accusation. Arthur’s deeply ambivalent feelings about the situation are reflected in his unwillingness to take any personal action to save Guenever and in his fervent hope that Lancelot will rescue her. The second step in the collapse of Arthur’s reign is Lancelot’s accidental killing of Gareth and Gaheris. Since the only account of their deaths comes from Mordred, we are never sure that Lancelot is their killer. Lancelot does not think he killed the two men, but he lies about why he was in Guenever’s room, and thus we cannot be certain that he is not simply concealing an evil deed. The ambiguity of Lancelot’s guilt makes Arthur’s position even more difficult. He cannot be sure that his best friend is guilty of killing Gareth and Gaheris, but he has to take decisive action against him.
The tone and pacing of the narrative in these last chapters differs from the tone and pace of the narrative in the earlier books of The Once and Future King. Unlike the slow, satirical, and sometimes frivolous description of the Wart’s childhood in Book I, or the fast-paced action scenes of Lancelot’s numerous quests and battles in Book III, the narrative in Book IV is jumpy, episodic, subtle, ominous, and mysterious. White constantly switches the narrative from one conversation to another, and we learn about events from a wide variety of perspectives. For example, we hear about Gawaine’s battles with Lancelot and Guenever’s flight from Mordred after they occur. White shapes the narrative in this way for a reason: the final book is extremely short, but its plot is complicated, so there is not enough space for the kinds of descriptions that we find in Book III. The length of the book is not the only reason for its choppy, fragmented style, however. Another reason is that the story is no longer trying to sidestep the issue of Camelot’s demise, since its downfall is now imminent and unavoidable. Instead, the novel describes what is left of Arthur’s empire in a manner that reflects how fragmented the kingdom has become.