In the bitter january of 1893, the German peace crusader Dr Otto Seligmann is blown to pieces in the Belvedere, his garden library at Chelsea. Detective Inspector Box and Sergeant Knollys interview Seligmann's associate, Count Cernzy, who reveals that Britain is infested by agents of the German war party. In the fog-shrouded garden of Seligmann's house, Box encounters Colonel Kershaw, the suave but sinister head of secret intelligence, who enlists his aid to search for Seligmann's house, Box finally discovers the secret of the Hansa Protocol, and the true purpose of the Belvedere explosion. The mission ends in a desperate confrontation on which the nation's future will depend.