The year is 1891. Sherlock Holmes and Professor Moriarty have perished at Reichenbach. But whilst England mourns the death of the great detective, the demise of his arch-enemy has not gone unlamented either. His younger brother – Lieutenant-Colonel James Artemis Moriarty – must break the news to the Professor՚s illegitimate daughter, Ada. To soften the blow, Artemis makes her a proposition; return with him to London and learn first-hand about the family business.

Ada gets more than she bargained for when she accepts. There՚s a power struggle within her father՚s syndicate between her Uncle Artemis and Colonel Sebastian Moran. Inspector Patterson of Scotland Yard intends to finish what Holmes started, and put the Moriarty Gang behind bars. And then there՚s a traitor in the gang, someone who sold the Professor out, and is prepared to kill Artemis and Ada to cover his tracks...