Martin Calvary is an agent with The Chapel.
That group is a small agency in the British Intelligence community which is so secret that not even the Prime Minister knows of its existence. Its mandate is to hunt down and eliminate enemies of the state no matter who they are or where they hide.
Calvary is very good at his job but as he rapidly nears the age of 40 at the time of the first recorded adventure, he is feeling each battle a lot more than the previous one. He still has what it takes to do the job but he is a realist who knows it cannot go on much longer. Many of his opponents seem to be younger each mission and while experience counts for a great deal, older muscles move slower.
Add to that the tiredness that his profession brings and it is easy to understand why Calvary would want out. Unfortunately, when you are as good as Calvary, is it not easy for your employer to just pat you on the back and say farewell.
More on this series when I have finished the books.