Mason Nash is an agent with MI6.
Was an agent, as far as he is concerned when we first meet him. As we are told in a blurb for that initial adventure, "Retired MI6 spy Mason Nash moved to a sleepy English town so he could leave his former violent life behind. He soon learns that past transgressions have a way of catching up with you."
Having put in his time and paid his dues with the British intelligence organization, and very tired of the sort of things he had to perform in those 'dues', he had put in his papers and happily gone into a non-violent retirement. He picked a "quintessentially English" town for his new non-spy life, albeit one with a less than tranquil sounding name of 'Devil's End'. His new vocation is that of history teacher at the local high school where they knew him as Edmond Green. And when he is not teaching or visiting the local pub for a pint of Newcastle Brown and a plate of Shepherd's Pie, he enjoys "sitting .. for hours on end" meditating and trying to forget the past.
Unfortunately, the past is not going to forget him and a very nasty, vengeful private intelligence organization called Tartarus has plenty of, to them, good reasons for wanting and needing Nash dead and they have some very capable people able to accomplish that. Well, not quite as capable as the late middle-aged Nash, as they will learn quickly. But that will not deter them and so he will have to temporarily un-retire to fight back.
Luckily for him, his old boss, Paul, learning of the new trouble, will send him help first in the form of an Aussie female MI6 operative named Eva Destruction and then her sometimes boyfriend, fellow agent Charles Bishop.