Sherlock Holmes yearns for a challenge when Lady Diana arrives, pleading for his help, holding herself responsible for the accidental death of a blackmailer.

Her predicament enthrals Holmes who knows the blackmailer to be an aide to Count von Runstedt whom he believes to be the head of a German spy ring. He and Watson probe this conspiracy and are plunged into a series of escapades which take them all over London and beyond involving them in burglary, treason and murder.

Holmes obtains a vital document needing decoding and acquires a scrap of paper indicating the whereabouts of top secret government documents which have gone missing, but this too needs unravelling.

It is also a tale of two women, from opposite ends of the social spectrum, who profoundly influence the course of events.

Set against a backdrop of the mounting tension in international affairs in the 1890s this is one of the most politically sensitive of Holmes's cases.