Martin Schuller is an agent with the U.S. State Department.
More specifically, he is part of their Counter-Intelligence Department. The time period we first following the recorded adventures of this man in 1939 and the United States has thus far managed to stay out of the trouble about to explode in Europe (the first case takes place in February and war declarations will not take place between England and Germany until the latter's invasion of Poland later that year in September). Though America is still very much in its prolonged isolationism, those running the State Department have known for some time how important being forewarned.
The days of Henry Lewis Stimson's policy of gentlemen not "reading each other's mail" may have passed some time before but we are told in our introduction to Schuller and his vocation, "the U.S. government, on principle, did not send spies into foreign lands; foreign governments, however, did not return the favor to the United States - creating the need for men such as Martin to identify and apprehend foreign agents on American soil. The Department of State's counterintelligence bureau sought out anyone using, or attempting to acquire, a fake U.S. passport - with particular focus on infiltrators from the Soviet Union and Germany. Martin worked for State as a senior analyst and sometime undercover operative."
Schuller is put on the German infiltrator issue not only because it is the most pressing problem of the day but also because he has a native-speaking ability with that Teutonic language. One interesting sidebar to his Pennsylvanian Dutch accent is something he learned during his two year sting as a Foreign Service Officer in Austria several year before - his "accent sounded vaguely Swiss to most German speakers". He is able to pass off any comment by claiming to be Alsatian.
One quite interesting tidbit from the adventures we follow Schuller on - he manages to acquire in the first mission an American adversary in the form of FBI agent Reginald Sloan, a particularly annoying fellow who disrupts an important rendezvous and follows that up with near constant harassment. The second assignment we join in on takes Schuller to South America and, unfortunately, so goes the troublesome Sloan. In an amusing note, however, on the third mission, Sloan does not play a part but Schuller has occasion to "channel" his own version of Sloan when another person interferes with Schuller assignment. The humor is not lost on Schuller.