Geoffrey Branscombe is an agent with British Intelligence.
At least he is part-time and that time is a bit more than he would like and considerably more than his wife would agree to. Unfortunately for the two of them, Bruce Denton, the man who finds the assignments that put Branscombe and his wife, Eve, into such dangerous situations has two powerful methods of getting the couple into harm's way. First, he is a charming man with such a winning and forceful personality that saying no is often impossible. Second, he presents to Branscombe such interesting matters of scientific curiousness that the man cannot resist. Denton himself is a hard man to pin down. He says he is the titular head of a small agency called S.O.R.T. which is a minor research group for the government yet he is brought in routinely on very high-profile cases and he jokingly admits he works at large for the Establishment, an nebulous group of very powerful people which has no official mandate but which gets things done nevertheless.
As a paying profession, Branscombe is a scientist and a very good one. The first of the two recorded adventures that Branscombe is involved in comes directly from his discovery of an anti-gravitational force known as the Nullifier. It is through this invention, and the efforts of unknown persons to steal the technology, that Branscombe comes into contact with Denton. When other attempts to uncover the truth prove inadequate, it is Denton that waves the lure of the hunt before the inquisitive man and reels him in.
Branscombe would be the very first person to chuckle at the idea that he is anything other than a quiet, unassuming researcher but his record during WWII lies at odds with that attitude. He claims early on that the rank he achieved during the War was more a case of no one else wanting the leadership role. And as for the DSO that he earned by braving the chance of capture by going behind enemy lines to prove that the Germans were withdrawing was really a case of not believing what Intelligence, a term he find amusing, believed was wrong but the only way of proving it was to go there himself. He says the medal was really a face-saving act by his superiors to cover their mistake.
These self-denigrating admissions do not get backed by his actions in the two adventures recorded, and the insinuation that there were at least a couple more undocumented. Branscombe is intelligent enough to not be foolhardy but he is not a shrinking violet and his athletic prowess first earned in his days at university gives him enough ability to get himself out of the trouble his curious nature brings to him.
His wife, Eve, is no wallflower herself being very intelligent and very determined. She makes a very good partner to Geoffrey though considerably less willingly.
Science plays a huge role in these two dramas and it is important to remember when reading them that at the time they were published, they were often categorized with some good cause as science fiction. And while some of the science mentioned in the books may not have been discovered yet, they are nowhere as far-fetched as seemed six decades ago.