The five-part romance which was composed by several authors--Cistercian
monks in France--though attributed to Walter Map
It forms what has, in the past, been called "The Vulgate Cycle
Its main innovation is its invention of the character of Sir Galahad, the saintly knight, replacing the naive and worldy Perceval.
The storyline can be derived from Robert de Boron's "lost" romance Le Roman du Graal. When I say it is missing, I mean that the "romance"--the poetic version--is missing. There exists a prose version, which verly clearly corresponds to the actions of the L-G:
Of these, the first survives, and forms a clear analogue to the Estoire del Saint Graal. The second and third only survive in prose form; the "Perceval" also contains a final portion called "Mort Artu." The Lancelot section is based on an early Lancelot du Lac, which still names Perceval as the Grail hero, and only goes so far as the death of Galehot.
Though the author at one point claims to be Walter Map, it also claims to be written originally by a monk in 717--this is highly unlikely.