Carruthers is looking forward to a relaxing holiday when he sets off for the Baltic Sea. Instead he finds himself at the centre of a complicated and deadly plot. Will Carruthers and his friend Arthur Davies discover why the sinister and mysterious Herr Dollmann is so interested in their little yacht, the Dulcibella? What does Clara Dollmann know about her father's plans? What secret scheme has brought Commander von Bruning of the Imperial German Navy to the East Frisian Islands? Can Carruthers and Davies solve the riddle of the sands?