In June of 1893, the future Nicholas II travels to London for a royal wedding, and he is the target of a professional assassin known only as La Sylphide. When Prince George of England is attacked by an armed anarchist who mistakes him for Nicholas, Barker and Llewelyn are brought in to help track down the assassin and others who might conspire against the life of the tsesarevich. Taking them through high society (including a masked ball at Kensington Palace) and the back streets, Barker and Llewelyn must solve the case of their lives—their 12th, following Lethal Pursuit—before the crime of the century is committed.