About the book (from Wikipedia):
This Idyll is told in flashback by Sir Percivale, who had become a monk and died one summer before the account, to his fellow monk Ambrosius. His pious sister had beheld the Grail and named Galahad her “knight of heaven”, declaring that he, too, would behold it. One summer night in Arthur’s absence, Galahad sits in the Siege Perilous. The hall is shaken with thunder, and a vision of the covered Grail passes the knights. Percivale swears that he will quest for it a year and a day, a vow echoed by all the knights. When Arthur returns, he hears the news with horror. Galahad, he says, will see the Grail, and perhaps Percivale and Lancelot also, but the other knights are better suited to physical service than spiritual. The Round Table disperses. Percivale travels through a surreal, allegorical landscape until he meets Galahad in a hermitage. They continue together until Percivale can no longer follow, and he watches Galahad depart to a heavenly city in a boat like a silver star. Percival sees the grail, far away, not as close or real an image as Galahad saw, above Galahad’s head. After the period of questing, only a remnant of the Round Table returns to Camelot. Some tell stories of their quests. Gawain decided to give up and spent pleasant times relaxing with women, until they were all blown over by a great wind, and he figured it was time to go home. Lancelot found a great, winding staircase, and climbed it until he found a room which was hot as fire and very surreal, and saw a veiled version of the grail wrapped in samite, a heavy silk popular in the Middle Ages, which is mentioned several times throughout the Idylls. “The Holy Grail” is symbolic of the Round Table being broken apart, a key reason for the doom of Camelot.