Inspector Ralph McGinniss and Professor Claire Lane are sent to investigate the link between the murder of a Police Constable on a lonely road, and the presence of German Zeppelin Commander Ludwig Mathieu left behind in England after the War.
Ralph McGinniss is the victim of a pickpocket on a Motorbus in central London and realises the woman responsible is connected to Jack the Top, a notorious thief and pimp.
Claire Lane is convinced the two are linked. When Ralph uncovers a plot to assassinate a well-known Judge from the High Court, the duo must act to discover what is the German planning, and how does the death of PC Bloomsdale, find the detectives investigating a fire in the village of Potters Bar.
Will they discover the truth in time or deal with the consequences?