In October 1889, the formula for a new and extraordinarily powerful explosive disappeared from the Ministry of Defense. Sherlock’s brother Mycroft, an eccentric but influential government servant, began a secret official inquiry into the theft, but his efforts were thwarted.

Before he could ask Sherlock to take up an independent investigation, Mycroft was seriously injured in an explosion that leveled the Diogenes Club.

Believing his brother had been the victim of a random gas leak, Sherlock retired disconsolately to 221B Baker Street. Dr. Watson eventually revived his interest by asking a single question: what if the explosion that injured Mycroft had not been an accident?