When Barker and Llewelyn are hired to find a girl from the upper classes who has gone missing in the East End, they assume her kidnapping is the work of white slavers. But when they discover five girls have been murdered in Bethnal Green, taunting letters begin to arrive in Craig's Court from a killer calling himself Mr. Miacca.

Barker fears that Miacca might be part of the Hellfire Club, a group of powerful, hedonistic aristocrats performing Satanic rituals. He must track the fiend to his hideout, while Llewelyn confronts the man who put him in prison.

Dodging muckrakers, navigating the murky Thames under cover of darkness, and infiltrating London's most powerful secret society, The Hellfire Conspiracy is another wild ride that "brings to life a London roiling with secret leagues, deadly organizations, and hidden clubs" (Detroit Free Press).