It is November 1890 and London is gripped by a merciless winter. Sherlock Holmes and Dr Watson are enjoying tea by the fire when an agitated gentleman arrives unannounced at 221b Baker Street. He begs Holmes for help, telling the unnerving story of a scar-faced man with piercing eyes who has stalked him in recent weeks. Intrigued, Holmes and Watson find themselves swiftly drawn into a series of puzzling and sinister events, stretching from the gas-lit streets of London to the teeming criminal underworld of Boston and the mysterious 'House of Silk' . . .