Standard counter-espionage fare fighting German saboteurs, featuring the introduction of a new agent working for Quayle, Michael Kells. In the movie version, the Germans were replaced with Russians.
Note: Peter Quayle is not mentioned specifically by name but a man that Kells refers to often as the "Old Man" is his boss and is the one that orders Kells to work with fellow agent Quelvada from a previous adventure who is almost certainly working for Quayle.