Three children on a holiday in Cornwall find an ancient manuscript which sends them on a dangerous quest for a grail that would reveal the true story of King Arthur and that entraps them in the eternal battle between the forces of the Light and the forces of the Dark.

"The priceless golden grail that Simon, Jane, and Barney Drew worked so hard to recover has been stolen by forces of evil. Great-Uncle Merry takes the siblings back to Trewissick in Cornwall, where he expects the Dark has hidden the grail. There, they are joined by Will Stanton, a mysterious boy with astounding powers. But there are more forces at play than they realize, and when the village women create the disturbing ritual creature called the Greenwitch-an...

Following a serious illness, Will is sent to stay with his uncle in the wild, bleak mountains of Tywyd. He is troubled by vague memories until he meets the mysterious Bran - and suddenly Will knows the task that lies ahead. With Bran's help, Will set outs to find the golden harp and awaken the six sleepers who must join the final battle.

On his eleventh birthday Will Stanton discovers that he is the last of the Old Ones, destined to seek the six magical Signs that will enable the Old Ones to triumph over the evil forces of the Dark.