Sherlock Holmes and The Hellfire Heirs

When Mrs. Hudson asks Holmes and Watson to find a missing girl, neither men expected what they would find.
Along with Lestrade, they are plunged into a case of abduction and murder. As they race from Marylebone, to Finchley, to Wapping, and back again, time is running out for them to solve the case without further casualties. With two of their own under threat, the pressure is mounting to catch and stop the group known as "Hellfire Heirs".
But just who are the shadowy men known as "The Hellfire Heirs"?