Merlin's powers are on the wane. Arthur's estranged half-sister Morgan le Fay launches a conspiracy against him, a plot focused on pursuit of the Holy Grail and involving their son Mordred and Arthur's best knight, Lancelot. With threats from inside the kingdom and from abroad, Knights of the Round Table begin to disappear. The one man who might solve the mystery of the Grail, Sir Gawain, is lost at sea on a journey to strange lands and other worlds.