Sam Farrell is an agent with the American Naval Intelligence.
Farrell, in performance of his job, carries a warrant card on which is printed: "NAVY DEPARTMENT, Washington, D. C. Be it known that the bearer of these credentials is SAMUEL FARRELL, a duly commissioned Lieutenant Commander of the United States Naval Intelligence. It is requested that all arms of the Service grant him full cooperation as may be demanded by him." It is signed by the Secretary of the Navy.
The time frame for the activities of this operative is the mid-1930's. The memory of the Great War is still around but receding while tensions are growing quite strong in the worries of a repeat. Several spheres of power are worrisome to the citizenry of the States, namely the rapid expansionism of Japan in the Far East, the increasingly worrisome fascism that has taken hold in Italy and Germany and is reaching into Austria, Czechoslovakia, and Poland, and the ever growing threat from the Soviet Union and its communism.
In that context the activities of agents of Naval Intelligence are particularly interesting and important. In that ere there were only two branches of the military - Army and Navy. As of yet, there were no American army troops serving outside the nation's borders therefore the need for intelligence operations were non-existent. Not so with the Navy which routinely traveled around the world showing the American flag and providing protection to the merchant fleet. The Navy was a constant target of saboteurs and foreign spies which in turn required a robust and ever-vigilant Intelligence department to protect the ships and crews wherever they were.
Answering that need was Sam Farrell, a man with a "long and lanky quietness" what was, according to Admiral Beasley, head of Naval Intelligence and his direct boss, "the best man we've got". Farrell was an officer in the Navy, holding the rank of Lieutenant Commander. Though we are not told a great deal about Farrell, we can deduce that he is likely in his mid-30s based on his achieving that rank and the fact that we are told he had a couple years before we meet him served for several years in Europe, notably the Balkans.