The Quest of the Holy Grail is a book which was presumably written in 1210, in Old French, by an unknown author; in 1969, medieval historian, scholar, and translator Pauline Matarasso translated the story into English. The book is the fourth volume of the Vulgate Cycle (also known as the Lancelot-Grail or Pseudo-Map Cycle), which consists of five books about the legend of King Arthur. Thus, The Quest of the Holy Grail is a story that follows the Knights of the Round Table as they search for the Holy Grail; the main plot, however, focuses on Sir Galahad.

The book opens with Sir Lancelot, who is asked to knight Galahad. Unbeknownst to both of them, Lancelot is, in fact, Galahad's father. Now a knight, Galahad is seated on the Siege Perilous, which is a throne reserved for the bravest of knights. After that, Galahad manages to draw the sword from the stone and defeats all of the knights in a sword-fighting tournament, except for Lancelot and Perceval. Thus, the knights organize a feast and everyone eats and drinks from the Holy Grail. However, under mysterious circumstances, the Grail suddenly disappears, and the knights go on a quest to find it.

Galahad gets a special shield which was, apparently, destined for him, and he goes on his own quest for the Grail. On the way, he manages to defeat the seven brothers and free the Castle of Maidens. He also defeats Lancelot and acquires a special sword. Meanwhile, Lancelot has visions of the Grail; however, he cannot do anything, as he is not pure and honorable. He acknowledges his sins and vows to change his ways.

We then learn from Perceval's aunt about the prophecy of the Grail, which says that three men will be able to find it—two virgins and one chaste man. The virgins are Galahad and Perceval, and Perceval almost gets tricked by a demon; the chaste man is Bors. Lancelot is also tested, but unlike Perceval, he fails, as he is blinded by lust and greed, and thus he is sent home. It seems that all the knights had to do to pass these tests on their quest for the Holy Grail was to remain pure-hearted and not lose their faith in God.

Galahad, Perceval, and Bors manage to get to the Grail castle, where Galahad meets Jesus. The three of them are given a vision of the Holy Grail, and they take the Grail to the city of Sarras, away from Britain, where Galahad is crowned king. A year later he is allowed to look into the Grail and asks for permission to die. God grants his wish and takes him to heaven, along with the Holy Grail. Bors returns to Britain to tell the story to King Arthur, and the knights' quest for the Holy Grail officially comes to an end.