In October 1889, a formula for a powerful new explosive vanished from the Ministry of Defense. Sherlock's brother Mycroft launched a covert government investigation — until he was gravely injured in an explosion that leveled the Diogenes Club. Believing it was a gas accident, Holmes retreated in grief to Baker Street, until Dr. Watson posed one unsettling question: what if the blast that nearly killed Mycroft was not an accident at all?