The plot is that a bored young man called Carruthers gets a telegram from a friend who invites him to join him on a yacht sailing along the coast of Germany and Frisia. This is in the north west of Europe on the coast of the North Sea.
Carruthers joins his friend, Davies, and gets much more than he bargained for. This is because Davies is an amateur spy, and is sailing around the coast in difficult waters to uncover secrets! The rest of the book gives their adventures together in their small boat.
The novel was written in 1903, not long before the First World War in 1914. This was a time when many people were worried about war and the plot hinges on secret military preparations for invasion.
There are a few expressions and attitudes in the book that seem dated - it is over a hundred and twenty years old. But the reason Riddle in the Sands is so exciting to read is that most of the book describes in great detail the day-to-day exploits of the two heroes in their tiny yacht, Dulcibella.
We have domestic details, how the two men cook and live in such a small space. There is also a huge amount of detail about how the men sail and navigate as they make their search. As the heroes sail in and out of little water channels in the sand and around islands, they must deal with the sea, the tides, the wind and the dangerous sands where the boat can get stuck (run aground).
The landscape takes on a life of its own!
This is an excellent example of an early spy story recommended for those who like sailing and adventure.
Idiom: to get more than you bargained for is to get more than you expected, perhaps something difficult.