Ted Dexter is an agent for the British Prime Minister.
The year is 1940, the location is the northern part of France, and the occasion is the major scramble back to the coast of the several hundred thousand troops that make up the British Expeditionary Forces that were on the main land to help the French stave off any advance westward by the Germans. Unfortunately, the Germans, seeing the trouble with the Maginot Line and with the very large contingent of enemy troops standing by, did an end-around through the Netherlands and Belgium, throwing everything into turmoil.
Dexter is a lance-corporal in an infantry platoon well inside France when we first meet him. He has no grand idea of being personally selected by Churchill and working on the sort of things he would be doing as his life, and the War, continues. What he is doing is trying to stay alive which is a full-time job because his Captain, a total twit named Pomeroy, has not had the decency to get himself killed yet but is trying to do the same for his 'men' because he is an officer and therefore much smarter than his NCO. Dexter is no saint himself but he tries to be politic when he points out the danger of the Captain's ideas and fails. So shortly after we say hello, Pomeroy and most of the troops have headed west and Dexter and a couple of his guys are left to provide defense.
The entire first adventure is a record of his travels, initially on his own after he gets separated from his men, towards the coast and hoped for passage back to England. Along the way he will 'pick up' a couple of men who will play a role in his further adventures as well as a couple of items collected near the end of his travels which will bring him to the attention of the Prime Minister and Brigadier Murphy, the man who will become Dexter's handler.
Dexter is not a typical operative. He doesn't want to be. He is a soldier, used to following orders, from Northern Ireland, a Protestant who grew up knowing he should distrust Catholics but having no real idea why (nor really did his parents) so decided that was a lot of nonsense. The Army was his home now mainly because he had nothing better to do and besides, he liked being around people and doing his small part here and there. He has had no training to be an unofficial go-to guy for Churchill but because he is a soldier and soldiers follow orders, when he is told he is now that guy, he inwardly sighs and shrugs and becomes that guy.