"The Riddle of the Sands" was published in May 1903, and it has probably sold more than two million copies in its lifetime. Its author Erskine Childers was infuriated when it was described as fiction, because for him the issue of a probable German invasion by sea was real and the danger obvious. Then, Childers meant it as a wake-up call to the British government to attend to its North Sea defences, "The Riddle of the Sands" accomplished that task and has been considered a classic of espionage literature ever since, praised as much for its nautical action as for its suspenseful spy craft.

The theme of "The Riddle of the Sands" is the relationship between Britain and Germany, in language, loyalty, and states of aggression. The novel shows what relations between Britain and Germany were like before the First World War. It develops the story of a sailing trip in the Baltic Sea, where two young adventurers-turned-spies uncover a secret German plot to invade England.