Dick Prescot, returning from his morning bathe at Blatchington Towers, finds a total stranger dead in the library, a revolver tied tightly into his hand with a piece of green string. Subsequently, it is discovered that the dead man had a profound interest in gaining possession of the world-famous Blatchington rubies, and that on the very night he was murdered an attempt was made to steal the rubies from Lady Blatchington's safe. Armed with these clues, Prescot attempts to solve the mystery by a study of the house-party, many of whom were behaving in a very suspicious manner; but finds his sympathies engaged with the very persons he had most reason to suspect. The imminent arrest of the most popular member of the party leads to the summoning of Mr. Wilson, the great detective....