All through time, the two great forces of Light and Dark have battled for control of the world. Now, after centuries of balance, the Dark is summoning its terrifying forces to rise once more...and three children find themselves caught in the conflict.

The Drew siblings--Simon, Jane, and Barney--are on a family holiday in Cornwall when they discover an ancient map in the attic of the house they are sharing with their Great Uncle Merry. They know immediately that the map is special but have no way of knowing how much. For the map leads to a grail: a vital weapon for the Light's fight against evil. In taking on the quest to find the grail, the Drews will have to race against the sinister human beings who serve the dreadful power of the dark--an adventure that puts their own lives in grave peril.