His father, a knight fleeing England, leaves the crippled young Hugh in the care of the monks of Glastonbury Abbey. In exchange for Hugh's care his father gives the Abbey a collection of books saved from their home library. Loving books, stories, and reading Hugh is put to work helping the Brother in charge of the scribes. Hugh soon makes a friend of Dickon, an oblate (sort of a monk in training) in a nearby monastery.

Dickon has always wanted to be a knight but was given to the monastery by his parents as a baby. Hugh has been warned by his father to say nothing of his background, family, or name except that he is named Hugh. This means he can't tell Dickon about any connections with the world of knighthood, but he unknowingly gives away enough that Dickon pieces it all together when Hugh helps a man, Jacques, who served his father, seek sanctuary.

Dickon takes Hugh to his secret place full of relics that relate to old saints, the holy grail, Joseph of Arimithea and possibly King Arthur. Glastonbury is said to be the site of old Avalon, the burial place of King Arthur where miracles happen and the past blends with the present.

Hearing strange sounds one day in their secret place they peek in and see a large, powerful older man seated in the chamber playing music and humming/droning. Dickon recognizes him as Bleheris, the mad monk. Ultimately these 3 together and with Brother John (I think his name is, who is training Hugh) as an unsuspecting collaborator, work to restore a broken book containing the tale of the holy grail.

All 4 characters contribute equally to their quest to learn the truth of Glastonbury, Avalon, and the fate of the holy grail and amazing things are seen and heard. Excalibur is found. The Abbey burns down. The origin, extent and purpose of the secret place and passages are determined. A child lady in waiting and her little dog are befriended. The broken book vanishes. Hugh becomes very ill after a vision of the burial of King Arthur having been lost in the swamp and overcome with exposure and exhaustion. And much, much more.