As London swelters through a relentless summer, Baker Street faces an extraordinary mystery. Armaments tycoon Charles Belladonna has died, leaving a fortune to an only son stolen from his cot years before. Now seven young men claim to be the missing heir. With the War Office taking a keen interest and dark forces determined to influence the outcome, Sherlock Holmes is called in to investigate. Yet the truth proves elusive. To uncover it, Mrs Hudson and her resourceful assistant Flotsam must deploy all their ingenuity before the false heir inherits far more than a fortune.