Stones of the Last Wall

The Foreign Intelligence Service of Russia has not faced a problem of this scale for a long time: there is information that someone is going to transfer comprehensive information about the entire Russian residency in Germany to the CIA.  

The disaster is averted at the cost of two agents' lives, but the spies learn that a new operation is being prepared.  

The case is assigned to Special Agent Drongo. 

During the investigation, he becomes convinced that the traitor once served in the East German security service "Stasi."  

But who is this mysterious "mole"?