Tempted by the idea of duck shooting, Carruthers joins his friend Davies on a yachting expedition in the Baltic.  But Davies has more on his mind than killing fowl.  As they navigate the waters and treacherous, shiftings sands on board the Dulcibella, Carruhers learns the real reason behind their trip - and how the safety of Britain depends on it.  Loosely based on the author's own experiences...