are now in great danger. He explains that some British artefact may have been hidden here long ago, and confirms that they will have dangerous grown-up rivals in its pursuit. So begins their quest for the Grail on behalf of the Light, which they have to achieve while being harried by Mr. Withers and his sister, who are agents of the Dark, desperate to stop them at any cost. Mother usually paints outdoors, and father goes boating, or both travel out of town. Meanwhile, the children investigate the meaning of the map, encouraged, yet warned and sometimes "guarded", by Great Uncle Merry. They learn to read the diagram and work out the clues on the map, but they must work out of doors, where each child has a nasty encounter with the Dark and their progress is easy to observe. While looking for the first clue Simon is chased by Mr. Hastings and Bill Hoover, Jr. After the second clue leads them to the headland at night Simon, Jane and Great-Uncle Merry are ambushed and almost caught by followers of the Dark. Merriman is misdirected out of town, but the children anxiously follow their ancient guide "over sea and under stone" without him. Barney is kidnapped by Mr. Withers and his sister Polly, and must be rescued. The children eventually follow the clues to a cave beneath the headland and discover the Grail. Unfortunately they lose an important metal case that was lodged inside the Grail, which contained a coded manuscript that is the key to deciphering the markings on the outside of the Grail. The children present the Grail to the British Museum and are given a cheque for it. The Grail is an object of debate among the scholars there because of the unknown markings. Barney begins to suspect something, reciting in his mind the name of the great-uncle Merry and eventually finding a link to Merlin. Characters Simon Drew: Eldest of three Drew children. Jane Drew: The middle Drew sibling. Barnabas Drew (Barney): The youngest of the siblings. Merriman Lyon (Great-Uncle Merry; Gumerry): An old friend of the Drew children's maternal grandfather and their mentor in everything. Captain Toms: Friend of Merriman, owner of The Grey House which the Drews rent, has a very smart red setter Rufus that helps the Drew children avoid danger and find the Grail. Mr. Withers and his sister Polly: Over-friendly tourists