The final tale of the Morte Darthur, “The Morte Arthur,” like “The Tale of Launcelot and Guenevere,” is based upon a part of the Vulgate Mort Artu and the corresponding part of the stanzaic Morte Arthur. The final tale relates the downfall of the Round Table and the deaths of most of the primary characters. It opens with a discussion between Gawain and his brothers, in which Agravain and Mordred, under the pretext of concern for their uncle's honour, wish to reveal the relationship between Lancelot and Guenevere. Gawain, Gaherys, and Gareth counsel against it and refuse to participate, but Agravain and Mordred persist. They suggest that Arthur test their accusation by spending the night on a hunting trip, during which Agravain, Mordred, and their knights will try to catch Lancelot and Guenevere together. They succeed in catching Lancelot unarmed in the queen's chamber, and when they reject Lancelot's offer to discuss the matter with the king upon his return, Lancelot fights his way out, killing all of his assailants except Mordred, who flees. Lancelot and his kinsmen retreat to Joyous Garde ahead of Arthur's homecoming.

Arthur sentences Guenevere to be burned at the stake. Lancelot and his kinsmen agree that he has a duty to rescue her, but in the melee he accidentally kills Gaherys and Gareth, who are both unarmed. Arthur besieges Joyous Garde, and Lancelot reluctantly defends himself. When the Pope intercedes, Lancelot willingly returns Guenevere to Arthur.