As Lancelot and Guenever sit in her solar, they sing together. They stop singing and begin to bicker, but the type of bickering lovers engage in. Lancelot asks if he can come to her room that night, Guenever is hesitant, scared of being caught. As they talk, neither of them notices Arthur's profile in the gathering twilight. Arthur leaves to find a page to announce him. The page clatters noisily up the stairs and alerts the lovers.
The two lovers are finally content with one another. What this scene illustrates, however, is the role Arthur has played in their relationship: forever on the periphery, but still conscious of it and choosing not to interfere. Arthur sees what is happening, and is willing for it to happen. In a sense, he has sacrificed his own happiness for that of his country.
Arthur looks older now, but with a noble oldness. The King is worried about the Orkney brothers and asks both Guenever and Lancelot to listen as he tells them something that he did wrong: Arthur tells them the story of how, before he met Guenever, he was seduced by Morgause (also his half-sister, although he did not know it) who then had his son. People at court had frightened him, telling him all sorts of prophecies. In the end, Arthur had ordered all the babies born at that time to be put on a ship and left to drown. However, Mordred his son had been saved.
It is strange that White did not describe the incident with the boat at the period in which it happened in Arthur's life. This suggests that White, in the same way as Arthur himself, did not want to besmirch Arthur's characters so that when he tried to implement his great idea, it would be as the law itself—idealistic, pure and just. Instead, this incident (revealed only now) highlights Arthur's fundamental humanity and thus the likelihood his almost inhuman reign will fail.
Arthur fears Mordred still bears him a grudge—which is why he tells the lovers this story. Guenever urges Arthur to imprison or kill Mordred for treason; Arthur will not because Mordred is his son. Plus, Arthur is King and must stand for law if he expects his people to act by the law.
This exchange captures the two aspects of Arthur's tragedy. That his own love for his son stops him from being able to intercede when that son is plotting against him. And that in creating a world of laws he has restrained himself from being able to intercede. The tragedy will be compounded when Mordred uses that same system of laws to trap Arthur, Guenever, and Lancelot.