After the Drew family arrives in Cornwall for a vacation, Jane, Barney, and Simon explore the village of Trewissick and the Grey House, where the family is staying with Great-Uncle Merry. The children soon discover a hidden door that leads to an attic, where they find a crumbling map. Soon after they are wrenched from their ordinary world into a strange and perilous world that exists parallel to the ordinary world, where the struggle between good and evil is more clearly evident. This map supplies the clues necessary for their quest to find the grail, an Arthurian relic that is a powerful weapon in the fight between the forces of good and evil, the Light and the Dark. Their quest is hazardous; the characters fighting for the Dark also want the grail, and the children mistake some of these characters for friends.

Great-Uncle Merry, a friend of the...