A very determined killer is seeking revenge on the four men responsible for his eight years in prison, and the murder of his wife and young son. The killer starts to put the knowledge he has acquired in prison into practice. As he begins to stalk his first victim Steve Burt and his team are asked to determine whether a serving RAF officer is a spy. If he is, then a kill order on him, issued by a clandestine MI5 department, will be enforced. Steve and his team grapple with MI5 and their demands, unaware that the officer has been targetted by an undercover assassin. Then the case involving the murder of two seemingly unrelated men is handed to them. With a lack of forensics, clues and witnesses and more murders, trying to track down an elusive killer is the most puzzling case the unit have had. This time Steve and the team have to use all their experience to resolve such difficult and demanding cases.