Lancelot and Guenever discuss their relationship. Lancelot wants Guenever to come with him to his castle, Joyous Gard, and live with him openly for the last part of their lives. Guenever refuses, not wanting to hurt Arthur. Lancelot accuses her of wanting two husbands. Arthur then comes upon them unobserved and hears their conversation. He backs out and sends a page to announce him. He greets them and wishes to tell them a secret. He explains once again the story of his birth and childhood. This is known to them, but they do not know that his half-sisters were the Cornwall sisters. He tells them that not knowing their true relationship, he had slept with his half-sister, Morgause, and sired a baby, Mordred. Lancelot refuses to find fault with his hero, saying that he did not know and Morgause was a witch anyway. Arthur refuses to let him talk about the mother of his only son that way, and he refuses to let Lancelot think him guiltless. He explains that knowing Mordred would be his downfall, he ordered all babies born at the time of Mordred’s birth to be placed in a boat and set out to sea. The boat landed on an island, but most of the babies drowned. Mordred survived and was returned to his mother. Lancelot sees Mordred as a threat and says that he should be executed. Arthur refuses to add more deaths to his conscience, even if it means his own doom.

The Orkney brothers gather in the Justice Room. Gawaine, Gaheris, and Gareth try to convince Agravaine and Mordred to give up their plan to accuse the queen of adultery and treason. As they are talking, Arthur enters. Mordred tells him that it is widely known that Guenever has been Sir Lancelot’s mistress for years. Arthur reminds him of Sir Meliagrance’s attempt to charge the queen with treason, but Mordred says that this was during the days of trial by combat. In Arthur’s new kingdom, trial by jury is the prescribed method. He says that he can bring evidence of the queen’s unfaithfulness that will stand up in court. Arthur warns him that if he fails to do so, he will use the same law to hunt both Mordred and Agravaine down.

Gareth comes to warn Lancelot about the trap to catch him with Guenever. Lancelot does not believe him, saying that Arthur would not allow such a thing and would not have gone off on a hunting trip if he knew it was planned. The signal comes from the queen, and Lancelot leaves Gareth to go to Guenever’s bed chamber.