"Over Sea, Under Stone" is the first book in Susan Cooper's classic fantasy series, "The Dark is Rising Sequence." Published in 1965, it follows the adventures of three siblings—Simon, Jane, and Barney Drew—during a summer holiday in Cornwall. They discover an ancient map that leads them on a quest to find the Holy Grail, hidden by their ancestor centuries ago. As they unravel the mysteries surrounding the map, they encounter both allies and adversaries in a battle of good versus evil. Cooper's series draws heavily from Arthurian legends and Celtic mythology, weaving together themes of magic, destiny, and the eternal struggle between light and darkness. "Over Sea, Under Stone" sets the stage for the rest of the series, introducing readers to the mystical world that the Drew children become increasingly entangled in as they uncover their family's connections to ancient powers.