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.

In 1890, the first Japanese diplomatic delegation arrives in London to open an embassy. Cyrus Barker, private enquiry agent and occasional agent for the Foreign Service Office, is enlisted to display his personal Japanese garden to the visiting dignitaries. Later that night, Ambassador Toda is shot and killed in his office and Cyrus Barker is discovered across the street, watching the very same office, in possession of a revolver with one spent cartridge.

In 19th-century London, Cyrus Barker and his associate Thomas Llewelyn are renowned private enquiry agents, successfully employed by the highest levels of Her Majesty's government as well as private citizens. Their success, however, has led to their acquiring a powerful group of enemies, many of whom are determined to have their revenge.

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.

At the request of Her Majesty's government, private enquiry agent Cyrus Barker agrees to take on his least favorite kind of assignment - he's to provide security for a secret conference with the French government. But shortly after the parties land at the island, Lord Hargrave is killed by a sniper shot, and the French ambassador's head of security is found stabbed to death. The only means of egress from the island - a boat - has been sent away, and the means of signaling for help has been destroyed.