In the attic of the big Grey House they are renting from Merriman's friend Captain Toms the children find an old manuscript.

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.

Soon the house is burgled, with attention paid only to the bookshelves and the wall hangings, and the children guess that someone else knows of and seeks the manuscript.

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.

Barney begins to suspect something, reciting in his mind the name of the great-uncle Merry and eventually finding a link to Merlin.

Among narratives intended for children it features fairly sophisticated use of English, with an extensive vocabulary and complex sentence structures.

[6] Joel Chaston writes, "Reviewers noted that the book begins as a mystery-adventure story and becomes a sort of morality tale full of mysticism and elements of Arthurian legend.

In a 1976 Horn Book essay, "A Second Look: Over Sea, Under Stone", Dwight Dudley Carlson argues that Cooper's superb abilities as a storyteller, the novel's clear delineation of good and evil, and the believability of the Drew children have contributed to its lasting success with young readers.

David Wood appeared as the storyteller, while dramatised sequences included Graham Crowden as Uncle Merry and Colin Jeavons as The Black Vicar.