Harry Maxim is a major with British Military Intelligence.
Maxim wondered why the Prime Minister wanted someone from Military Intelligence to fill a new, unspecified position at 10 Downing Street. He also wondered why he was the one chosen. He was certainly good at his job, experienced and with a clean record, but that hardly answered the question. He never did find out.
But he was chosen and reported for duty in the civilian world of Intelligence, acting on the PM's behalf and doing things that would never get recorded in any official record.
Not part of the Old Boy network in the military, Maxim had risen as far as he was likely to unless a new war broke out somewhere and there were none on the horizon. Plus there were the mysterious events which occurred after his wife was killed in a plane explosion, events that left a fair number of terrorists dead, culminating in an injury that left him in the hospital for six months. A lot of questions were asked and no one was willing to answer any of them, least of all Maxim. Now healed and in superb shape, Maxim took the new posting as a matter of course. Since no one was telling him exactly what he should be doing, he continued to do what he did at M.I., check into things that looked interesting.
Outside of work, Maxim's life is pleasantly plain, if quite lonely with the loss of his wife. He is left to raise his son, age 10 or so, by himself though he does have his parents to call upon when the need arises. And there is his sister with whom he still has routine sibling squabbles.
As the series progresses, his assignment changes but he remains working in one capacity or another with the same group.