Sir Modred, Sir Aggravayne, and their 12 companions catch Sir Launcelot and Queen Gwynevere together and accuse them of treason. Launcelot kills all of the knights except Modred, then flees to the Joyous Gard.
Sir Launcelot saves Queen Gwynevere from being burned at the stake and takes her to the Joyous Gard. King Arthur and Sir Gawain besiege Launcelot.
The Pope stops the fighting and commands Launcelot to return the Queen. Once he has done this, Arthur banishes Launcelot according to Sir Gawain's wishes. Launcelot then flees to France.
Arthur and Sir Gawain follow Launcelot to France and besiege him again. Gawain is injured in single combat with Launcelot. Meanwhile back in Britain, Sir Modred usurps the throne and makes unsuccessful overtures to Gwynevere.
Arthur returns to Britain and fights Modred for the throne. Three battles are fought, and Sir Gawain, Sir Modred, King Arthur, and many others are killed. Launcelot arrives to help Arthur, but he is too late.
Queen Gwynevere becomes an abbess once she hears of Arthur's death. Eight of Launcelot's knights, including Launcelot himself, become hermits with the Archbishop of Canterbury at the hermitage where Arthur is buried. Gwynevere dies and is buried next to Arthur. Six weeks after Gwynevere's death, Launcelot dies of grief, but is accepted into heaven.