Now that he's the powerful head of Britain's Special Branch, some people fear that Thomas Pitt may have been promoted beyond his abilities, in this 27th mystery starring the indefatigable investigator and his resourceful wife Charlotte (following Treason at Lisson Grove). Pitt's own self-doubt is fueled by rumors of a plot to blow up connections on the Dover-London rail line, on which Austrian duke Alois Habsburg is soon to travel. But why destroy an entire train to kill one obscure Austrian royal? Are the rumors designed to distract police from an even more devastating plot?