When a bomb destroys the recently formed 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, Barker and 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.

Their adventures take them from a lighthouse on the craggy coast of Wales to a Liverpool infested with radicals, and even to the City of Light, where Llewelyn goes undercover with Maire O'Casey, the alluring sister of an Irish radical. Llewelyn again finds himself put to the test by his enigmatic employer, studying the art of self-defense and the brutal sport of hurling -- and, most dangerous of all, being schooled in the deadly science of bomb making.