When a student bearing a striking resemblance to artists' renderings of Jesus Christ is found murdered -- by crucifixion -- in London's Jewish ghetto, 19th-century private detective Barker must hire an assistant to help him solve the sinister case. Out of all who answer an ad for a position with "some danger involved," the eccentric and enigmatic Barker chooses downtrodden Llewelyn, a gutsy young man whose murky past includes recent stints at both an Oxford college and an Oxford prison. As Llewelyn learns the ropes of his position, he is drawn deeper and deeper into Barker's peculiar world of vigilante detective work, as well as the dark heart of London's teeming underworld. Together they pass through chophouses, stables, and clandestine tea rooms, tangling with the early Italian mafia, a mad professor of eugenics, and other shadowy figures, inching ever closer to the shocking truth behind the murder.