Captain Forsyth and Sergeant McLean, Spy Hunters, are agents of British Military Intelligence.
At least that is the implied state at the start of the series but this quickly changes with Forsyth being shown to be just a very dependable, often called upon army leader and Sergeant McLean is just one of the NCO's that Forsyth might command. It is fascinating, in fact, that after just a couple of initial adventures, the good and trusty sergeant is never mentioned by name despite having equal billing on the masthead.
Both Forsyth and McLean are members of the Seaforth Highlanders, a regiment that came into existence in 1881 and would last as a distinct unit until 1961 when it merged with another regiment to become the Queen's Own Highlanders and merged with yet another one in 1994 to become the Highlanders. Its name, according to Wikipedia, comes from its founder, Kenneth Mackenzie, 1st Earl of Seaforth.
Forsyth is depicted as a man in his early 30s with dark hair cut in standard officer style (short but not acutely so) and wearing a neatly trimmed mustache. He is ramrod straight in posture and is very much a military man with his ability to bark out orders with decisiveness as well as take orders from his superiors without question or comment. He favors a pipe but otherwise has no obvious vices. McLean, in the very few times we see him, sports a kilt and a short sleeve blouse with his head topped normally with a Balmoral bonnet. Like his boss, he has a mustache (thinner) and enjoys a pipe now and then.
On odd but interesting point to mention is that though there are 15 adventures in the recorded series, we never learn the first names of either of the stars (except for Forsyth's use of the normal nickname of "Mac" for McLean.
As indicated by the main name of this series, the purpose for the two men should be seeking out and removing enemy spies. Except for the odd mission here and there, looking for spies is never the primary objective though considering the sort of adventures Forsyth has he does come across more than his share of sneaky individuals. Spy hunting, though, is not really happening.
What is going on is fairly normal, reasonably interesting military operations fighting to preserve Britain's control of its domain in three basic regions, India, Palestine, and China, before heading to France to take part in the new struggle against Nazi Germany. Rebels, insurgents, unhappy local populace, and others are the sort of people that Forsyth has to deal with as he goes about his duties following orders and keeping the peace.