Three years after Sherlock Holmes and William James Moriarty returned from the Final Problem, MI6 has been assigned to gather intelligence on a mysterious new street drug. As the rumblings of a war between the world's great powers loom in the distance, this highly addictive drug is quietly poisoning the British people. Now, Sherlock's latest case has landed him on the same trail as MI6, and William will have to make a decision about whether to come out of retirement and join the investigation.