Over Sea, Under Stone is a young adult fantasy novel published in 1965 by the English author Susan Cooper. It is the first of five books in Cooper's
The Dark is Rising series. It tells the story of the three Drew children who while on holiday discover secrets related to the legend of King Arthur.
The Drew family has three children: the eldest is Simon, the middle child is Jane, and the youngest is Barnabas, or Barney. With their parents, they meet up with their "great uncle" Merriman Lyon--an old family friend--for a holiday vacation in Trewissick, a fictional fishing town on the coast of Cornwall in England. Shortly after arriving, the siblings explore the attic of the old Grey House their parents are renting from "Captain Toms," an old friend of Merriman. There, they find a very old looking manuscript. The writing is faded and illegible, but the siblings make out a drawing of a nearby coastline, meaning that the old sheet of paper is a map. Meanwhile, Barney interprets some of the symbols on the map as references to the legend of King Arthur and the Knights of the Roundtable. Excited by their discovery, the siblings resolve to keep the map a secret from the adults.
The next day, Mr. Withers and his sister Polly invite the family to go yachting with them. While Simon and Barney jump at the opportunity, Jane is suspicious of the Withers' intentions for vague reasons and decides to stay home while the others go sailing and fishing. Left to her own devices, Jane continues to explore Grey House in search of further juicy discoveries. It isn't long before she finds a Trewissick guidebook published by the local vicarage. Within the guidebook's pages, Jane discovers a drawing of the coastline that closely resembles the map she and her siblings discovered on the previous night.
Excited by her discovery and curious to learn more, she rushes out to the nearby vicarage and asks the vicar about the guidebook. Despite what the guidebook says, the vicar claims he did not write it. He does however ask a number of probing inquiries that further inflame Jane's already suspicious nature.
One day not long after that, a burglar breaks into Grey House. Curiously, the burglar ignored traditional valuables like jewelry or cash and instead went straight for the bookshelves. The parents are mystified by this, but Simon, Jane, and Barney believe they know the truth: The burglar was looking for the old map manuscript. Now that they realize the value of the manuscript in their possession, the siblings decide it's time to come clean to Merriman about everything. After hearing their story, Merriman confirms what they were all thinking: The manuscript is a treasure map. Moreover, the treasure in question is none other than the Holy Grail for which Arthur's knights famously quested. And finally, Merriman admits that the children are in great danger, for there are forces of darkness--"The Dark," as he terms it--after the Grail who will stop at nothing to retrieve it. And while Merriman worries for their safety, he also encourages them to seek out the Grail so they can claim it on behalf of "The Light," as the alternative is too dark a possibility to bear.
And so each day--when Mr. Drew is out boating and Mrs. Drew is painting landscapes somewhere--the siblings puzzle out the clues on the map. Little do they know that the whole time they are being watched by followers of the Dark, including Polly and Mr. Withers, toward whom Jane felt such strong suspicions earlier. In fact, nearly everyone else they meet seems to be an agent of the Dark. That includes the housekeeper at Grey House, Mrs. Palk; Palk's nephew, Bill Hoover Jr.; and Mr. Hastings, who had been impersonating the vicar in order to pump Jane for information.
The situation really intensifies after Merriman is ambushed and forced out of town by agents of the Dark. These same agents even kidnap Barney in order to lead the children into a trap. Fortunately, they successfully free Barney and the trio escapes the clutches of the Dark long enough to follow their map "over sea, under stone" to the hiding place of the Holy Grail. There is also a small metal piece inside the grail which Barney infers may hold a clue to decipher the markings on the outside of the cup. Unfortunately, while managing to escape from the agents of the Dark, the children misplace the small metal piece.
Rather than be tempted by the grail's undeniable power, the children understand--like Indiana Jones--that the artifact belongs in a museum. Luckily for them, the British Museum cuts the sibling trio a generous check in return for the grail. As the children walk off satisfied with the resolution of their adventure, Barney has an epiphany: Merriman is Merlin, the wizard from the King Arthur legend.