So it mishapped that Sir Agravaine on a day said openly, so that many knights might hear, that the friendship between Sir Launcelot and the Queen was a disgrace to knighthood and a shame to so noble a king as Arthur. But Sir Gawaine would not hear any of these tales nor be of Agravaine's counsel; moreover he charged his brother to move no such matters afore him, for he wist well what mischief would come, should war arise betwixt Sir Launcelot and the King, and he remembered how ofttimes Sir Launcelot had proved his goodness and loyalty by knightly deeds. Also Sir Gaheris and Sir Gareth, two other brethren, would know nothing of Agravaine's base accusation.
But Sir Mordred, the fifth of the brethren, sons of the Queen of Orkney, the which had mocked the good Percivale when first he came to the court, and who had ever been jealous and ready to think evil of another, joined with Sir Agravaine. Therewithal they three, Sir Gawaine, Sir Gaheris, and Sir Gareth departed, making great dole over the mischief that threatened the destruction of the realm and the dispersion of the noble fellowship of the Round Table.
So Sir Agravaine and Sir Mordred came before King Arthur, and told him they might no longer suffer Sir Launcelot's deeds, for he was a traitor to his kingly person. But the King would believe nothing unless he might have proofs of it, for, as the French book saith, he was full loath to hear ill of a knight who had done so much for him and for the Queen so many times that, as was fully known, he loved him passingly well.
Then these two brethren made a plot for taking Sir Launcelot when in the Queen's presence, and bringing him dead or quick to King Arthur. So on the morn Sir Agravaine and Sir Mordred gat to them twelve knights and hid themselves in a chamber in the castle of Carlisle, where Queen Guenever was; thus they plotted to take Sir Launcelot by force, if she should have speech with him. Sir Launcelot was no coward, and cared not what liars said about him, since he wist his own good will and loyalty. So when the Queen sent for him to speak with her, he went as true knight to the castle, and fell into the trap that was set for him. In the battle that followed he was hard bestead, but slew Sir Agravaine at the first buffet, and within a little while he laid the twelve chosen knights cold to the earth. Also he wounded Sir Mordred, who, when he escaped from the noble Sir Launcelot, anon gat his horse and rode unto King Arthur, sore wounded and all bleeding.
Then he told the King how it was, and how they were all slain save himself only. So the King believed Sir Mordred's evil accusation true, and he said: "Alas, me sore repenteth that ever Sir Launcelot should be against me. Now am I sure the noble fellowship of the Round Table is broken for ever, for with him will many a noble knight hold. And now it is fallen so that I may not keep my honour unless the Queen suffer the death."
So then there was made great ordinance that the Queen must be judged to the death, for the law was such in those days that whatsoever they were, of what estate or degree, if they were found guilty of treason, there should be none other remedy but death. Right so it was ordained for Queen Guenever, and she was commanded to the fire, there to be burned.
King Arthur prayed Sir Gawaine to make himself ready in his best armour, with his brethren Sir Gaheris and Sir Gareth, to bring the Queen to the fire, there to have her judgment, and receive the death. But Sir Gawaine ever believed Dame Guenever guiltless of the treason charged against her, and he would never have it said that he had any part in her shameful end. Sir Gaheris and Sir Gareth also were loath to be there present, but they were young, and full unable to say him nay. "If we be there by your straight commandment," said they, "ye shall plainly hold us excused though we go in peaceable wise, and bear none harness of war upon us."
So the Queen was led forth without Carlisle, and she prepared herself for death. There was weeping and wailing and wringing of hands of many lords and ladies, and few in comparison there present would bear any armour for to keep order.
Anon as the fire was to be lighted, there was spurring and plucking up of horses, and right so Sir Launcelot and his followers came hither, and whoever stood against them was slain. And so in this rushing and hurling, as Sir Launcelot pressed here and there, it mishapped him to slay Gaheris and Gareth, the noble knights, for they were unarmed and unaware. In truth Sir Launcelot saw them not, and so were they found dead among the thickest of the press.
Then when Sir Launcelot had thus done, and had slain or put to flight all that would withstand him, he rode straight unto Dame Guenever, and made her to be set behind him on his horse, and prayed her to be of good cheer. Wit ye well the Queen was glad that she was escaped from the death, and then she thanked God and Sir Launcelot.