The story begins with the conspiracy against Lancelot and Queen Guinevere by Sir Mordred, Arthur's son and nephew from his half-sister Morgause, as well as Sir Agravain, also a knight of the round table. This plot against Guinevere and Lancelot is brought about through hatred by Mordred. He went to Arthur claiming that the two were having an affair and intended to catch them in less favorable circumstances. Having unbelief that his Queen and his best Knight would do such an act, Arthur agrees to Mordred's terms. When Mordred and the other Knights of the Round Table discover Lancelot in Guinevere's room they mean to take him prisoner to take before the King, but he escapes, but not before killing all the knights and wounding Mordred. After Lancelot escapes Guinevere is to be put on trial for committing treason against the King; she is found guilty. Before her execution Lancelot, along with his other knights, rescue Guinevere. During the rescue Lancelot, unbeknownst to him, killed Sir Gawain's brothers, Sir Gareth and Sir Gaheris. This occurrence causes Gawain to fall into mourning telling Arthur that he must wage war against Lancelot.
This war sends Arthur and Gawain to leave Mordred in charge of Camelot, who had an insidious plan for the kingdom. While away, Mordred, wanting Camelot for himself, falsified papers to declare that Arthur was killed in battle and became king over the land. When Arthur heard about this he set sail to wage war against Mordred. Unfortunately, before he obtained word of Mordred's betrayal, Gawain and Lancelot combated hand-to-hand, leaving Gawain terrible wounded which led to his death. In a warning by Gawain's ghost, telling him that if he fights Mordred he will die, Arthur asks Mordred for a trues to be signed, but due to misunderstandings war continues which causes Arthur to kill Mordred, but before he dies he fatally wounds Arthur.
Arthur tells his knight, Sir Bedivere, to take Excalibur and toss it in the lake where the Lady of the Lake gave it to him. Not wanting to waste such an exquisite weapon he hides it, but is found out by Arthur due to the fact that when inquiring about what happened when he threw it into the lake and gets the answer, "nothing", he knows it was not done. Finally, Bedivere throws it into the lake and is caught by the Lady of the Lake and brings news of this back to the dying Arthur. Three ladies came to take Arthur to Avilion where he might heal. After he was taken away it was told to Bedivere that a man, whom it is believed to be Arthur, was brought to the Bishop of Canterbury, but was not identified and was buried. Sir Lancelot brought Queen Guinevere's body to King Arthur's grave to be buried; soon after Lancelot died as well.