When Cyrus Barker and Thomas 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.

Private enquiry agents Cyrus Barker and Thomas Llewelyn receive in the mail an unexplained key stamped with the letter Q. Barker, recognizing it for what it is, uses the key to unlock an anonymous door in the alleyway, which opens to an underground tunnel leading to Downing Street. The Prime Minister has a small task for Cyrus Barker. A Foreign Office agent stole a satchel in Eastern Europe, but was then himself murdered at Charing Cross. The satchel contains a document desperately wanted by the German government, but while the agent was killed, the satchel remains in English hands.

When an Italian assassin's body is found floating in a barrel in Victorian London's East End, enquiry agent Cyrus Barker and his assistant Thomas Llewelyn are called in to investigate. Soon corpses begin to appear all over London, each accompanied by a Mafia Black Hand note. As Barker and Llewelyn dig deeper, they become entangled in the vendettas of rival Italian syndicates - and it is no longer clear who is a friend or foe.

When a bomb destroys the Special Irish Branch of Scotland Yard, all fingers point to the increasingly brazen factions of Irish dissidents seeking liberation from English rule. Volunteering their services to the British government, Cyrus Barker and Thomas Llewelyn set out to infiltrate a secret cell of the Irish Republican Brotherhood known as the Invisibles. Posing as a reclusive German bomb maker and his anarchist apprentice, they are recruited for the group's ultimate plan: to bring London to its knees and end the monarchy forever.

Some years ago, Cyrus Barker matched wits with Sebastian Nightwine, an aristocrat and sociopath, and in exposing his evil, sent Nightwine fleeing to hide from justice somewhere in the far corners of the earth. The last thing Barker ever expected was to encounter Nightwine again - but the British government, believing they need Nightwine's help, has granted him immunity for his past crimes, and brought him back to London.