Nick Parker is a part-time agent with the CIA.
Part-time in that being an operative is not his real job and he only becomes one as part of a couple (so far) events which puts him working with on on behalf of the Agency. It is not his intent to become a full-time agent, at least not yet.
The activities we follow Parker in take place in the mid-to-late 1970's.
When we first meet Parker he is a staff sargeant in the U.S. Air Force, specifically "the USAF "B" Flight Chief equivalent with a force of twenty-five Combat Security Police" under his leadership. From what I can tell, that means he is in charge of two dozen airmen whose job it was to provide security over government facilities. In this case, these facilities were in Thailand as the final couple of years of the Vietnam War. It seems the CIA had an operation going on there and Parker and his people would be called upon to lend assistance.
The two adventures we have of Parker tell of two major parts of his life. The first is the horrific events he lives through in Indochina and the severe ramifications he will have to deal with as a result of them. The second takes place a couple years after those days had ended and he finally was able to get his life back together, which is when the Agency comes calling again.
One thing we learn quickly about Parker is that he is very much a Christian and a man who wants to do the right things in his life. He will be faced with a good number of temptations and succumb to more than a couple of them, as is so easy in war to do. As a result, besides having to deal with piecing his physical body back together, he also has to deal with his spiritual well-being. Luckily he has at his side during a good portion of this rebuilding the love of his life, Eliz.