It is difficult to appreciate The Natural without some knowledge of the mythological traditions behind it. The most important of these are the legends of the Waste Land and the Fisher King. Malamud loosely based his novel on the story of Sir Perceval and his quest for the Holy Grail, originally recorded in the eleventh century by the French writer Chrétien de Troyes.
In Chrétien’s story, Perceval starts out as a country bumpkin, much like Roy. Raised in the forest by an overprotective mother, he has little knowledge of manners or chivalry. One day, Perceval meets several knights of King Arthur, and he immediately wants to join them. He goes to Camelot, but Arthur refuses to make him a knight until he proves himself. Perceval goes out to do so, and he proves his worth by winning many matches; he turns out to be a surprisingly good knight. Perceval meets a knight who arms him and teaches him about chivalry, particularly the idea that he should not chatter, and should instead remain quiet most of the time. Perceval plans to return to his mother and show her his new skills, but he is waylaid by an infatuation with a woman named Blancheflor.
Finally, one day, Perceval comes upon a strange castle. Inside is an old man, who presents Perceval with a fine sword. Perceval then witnesses a strange procession: several youths enter the hall carrying a bleeding lance, golden candelabra, and a golden grail. Perceval, remembering the advice of the knight who instructed him, decides to stay quiet and wait to ask the old man about the mysterious procession until the next morning. When Perceval wakes up, however, he finds the castle and its inhabitants have disappeared. He rides on and meets a woman who tells him that if he had only asked the right questions, he would have learned about the lance and the Holy Grail and could have healed the Fisher King, and thus also the Waste Land.
Though Chrétien died before he finished the story of Perceval, scholars are reasonably sure, based on the sources from which Chrétien worked, that Perceval returned to the Fisher King and, swallowing his pride, asked the questions necessary to obtain the Grail and heal the King.
via SparkNotes: The Natural: Themes, Motifs, and Symbols.