England, 1913. The horizon is darkening with the prospect of war when Dr John Watson confronts embattled Prime Minister Jasper Cranfield with evidence of a crime that has been spread over decades.
A sequence of interconnected mysteries, from the fate of a music hall murderer, to a man driven mad by a worm unknown to science, to a bizarre and sinister society which meets in a furniture warehouse - these untold cases hint at something larger. What is the purpose of the enigmatic organisation which calls itself 'The Lighthouse'?
How much does the Prime Minister know? Is Sherlock Holmes truly retired? And what final confrontation do they face?