Until Will's time the Old Ones have fought a holding action, keeping the Darkness at bay. Will's task is to gather things of power that the Light may use to finally drive the Dark from the world. He will not be alone in this task though. The first of the old ones (easily seen as Merlin), Merriam Lyon in our world, is there to teach and guide him as much as possible, but it's four other young people around his age that will have key roles in assisting him.

Barney, Jane, and Simon Drew are regular children on holiday with their Great Uncle Merriam in the fishing town of Treswick in Cornwall England in the first book of the sequence, Over Sea, Under Stone, and they recover the first thing of power. An ancient Celtic Chalice that is the key to deciphering a scroll that will provide the Light with vital information needed to help overcome the Dark.

The first time they and Will meet is in book three — The Greenwitch — in which the four work with Merriam to recover both the Chalice and the lost scroll. Back in the same village of Treswick that everything started for the Drews in book one, Cooper incorporates one of the old fertility rites of the Cornish coast into the storyline.

The Greenwitch of the title is a construct of Hawthorn and Rowan branches made into a giant figure, weighed down with rocks, and offered to the sea in the springtime to ensure good fishing and crops. As the fishermen head out to sea, the women of the village gather on the headland to build the Greenwitch. When the men come home in the morning, safe and with their holds full of fish, they join the women on the headland and push the offering out to sea.

With the reluctant help of the Greenwitch and the White Goddess of the Sea, Tethys, the Drews, Will, and Merriam are able to recover the Chalice, the scroll, and decipher the secret writing on the cup. The next stage of Will's journey will be in the oldest part of Great Britain: Wales.

The power of the Dark can't directly harm or kill an Old One, but it can use a variety of forces against them. At the beginning of The Grey King will is beset with a fever that comes close to killing him. Although he does recover in the end, he discovers that he's forgotten the instructions the Grail had given him for completing his task in Wales.