|US Cover Artist:||?|
|UK Cover Artist:||Margery Gill|
Simon, Jane and Barney Drew visit the seaside village of Trewissick in Cornwall with their parents, to stay with "Great-Uncle Merry". They stay in The Grey House, which belongs to a Captain Toms, who has left behind his red setter Rufus for them to look after.
One day, while playing indoors, the children find an attic and Barney discovers a map and manuscript hidden under a floorboard. The children examine it, and Barney realises that the map refers to King Arthur and his knights. The children decide to keep the discovery to themselves.
The family are visited at the Grey House by a very friendly Mr. Withers and his sister Polly, who invite them to go fishing on their yacht. The boys are thrilled, but Jane feels suspicious and declines to join them. While Jane is alone in the Grey House, she finds a guidebook to Trewissick in an old trunk, written by the local vicar. She realises that the map in the guidebook is similar to the secret map, but also different somehow, so she decides to visit the vicar. The man at the vicarage is not the writer of the guidebook, but he offers to help Jane. He asks some probing questions which arouse Jane's suspicions again, and she decides to return home.
That night, the Grey House is burgled while everyone is sound asleep. The children decide it is time to confide in Great-Uncle Merry. Up on the headland they show him the map and he tells them that it is a copy of an even older map which shows the way to a hidden treasure and that the children are now in great danger. And 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.
The children work out the clues on the map and have a number of run ins with the Dark individually and as a group. 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 Dark followers. Afterwards Barney is kidnapped and must be rescued. The children eventually follow the clues to a cave off the headland and discover the grail. Unfortunately they lose an important scroll from inside 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 check for it. The grail is an object of hot debate among the scholars there because of the unknown markings.