After surviving an attempt on his life in Paris, Sherlock Holmes returns to London, where the great detective is almost immediately enlisted by his brother to secure the safety of the delegates at a top-secret peace conference. With Europe a veritable powder keg in the years before World War I, Holmes understands any misstep on his part could prove fatal and possibly plunge the continent into war.

However, after pledging to do his utmost for "King and Country", Holmes suddenly finds himself overwhelmed by an onslaught of cases. An old friend requests his assistance in recovering a priceless manuscript which has gone missing from the British Museum. Accused of accepting bribes from a smuggling ring, Inspector Lestrade, who has been suspended from Scotland Yard, turns to Holmes for help. Add in a beautiful newlywed who claims her husband is trying to murder her, and it is easy to see why Watson compares the tasks confronting his friend to the Labours of Hercules.

From a secret pied-à-terre in the City of Lights, to the Rare Book Room in the British Museum, to the Whispering Gallery in St. Paul's Cathedral, to the waterfront along the Thames, Holmes and Watson find themselves on the trail of an elusive quarry for whom murder is merely another move in an elaborate game of cat and mouse.