For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Arthur drives the pagans out of Britain, unites the warring petty kings, and establishes the fellowship of the Round Table. His marriage to Guinevere solidifies his power.
Act II
Arthur’s knights embark upon various adventures. Gareth and Geraint win wives for themselves. Lancelot wins the tournament of the ninth diamond but refuses the love of Elaine. The wily Vivien causes the deaths of Balin and Balan and imprisons Arthur’s advisor Merlin in an oak tree. Poor young Pelleas goes mad when Gawain and Guinevere’s betrayals end his faith in the integrity of Arthur’s court. Many of Arthur’s knights undertake the Grail Quest, but only Lancelot, Bors, and Percivale have much success at it, with only Galahad achieving it fully.
Act III
Mordred discovers Lancelot and Guinevere alone together in her tower, exposing their affair. Lancelot flees to his lands, where Arthur pursues him and engages him in battle. Taking advantage of the discord, Mordred ranges a faction of Arthur’s knights against him.
Arthur pursues them to the western shore of Lyonesse, where he kills Mordred and receives his own death wound. After ensuring that Bedivere returns Excalibur to the lake, he boards a barge piloted by three mysterious women, heading out of this world into Avilion.