Alex Swan is an agent with MI-5.
As we meet this highly respected and long serving member of Britain's Internal Security department, he is the senior member of what might best be described, had it been in the civilian world, as a private investigation firm. It is addressed as the "Services Investigations Department" (SID) and is said to be "an independent department" working with and for but apparently not in MI5. The exact details are a mite mysterious, which seems rather fitting considering the setting, but Swan takes cases connected to national security and looks into them much as a private eye would but with a good deal of authority.
In the first case we follow with Swan, his former position in MI5 is a tad murky though definitely of a high echelon. In the second we learn it was indeed a major position as he was described as a former "Head of Espionage and Counter-Espionage at MI5". His current occupation came about when he ""was asked to set up an independent department while still with the Security Service" and hence created SID. It appears that he is free to work on whatever he feels necessary but the matters we follow with him all deal in one way or another with matters of national security.
Swan is a "tall thin man" in his late forties, with "a clean shaven, thin and gaunt looking face with hazel coloured eyes and a small mole at the side of his nose". He sports "salt and pepper coloured hair what was completely grey at the temples". He is single and, it would seem, has always been so though he does speak now and then of a lady friend. Since she is never identified nor described, it might be a single person or a series of friendships. Whichever might be the case, his true passion is without a doubt his work and throws himself into every case with considerable vigor.
Working closely with Swan and making up the other half of the partnership is Arthur Gable, "a largely built, balding elderly gentleman", prone to wearing dark grey pinstripe suits and giving names to the notebooks he routinely carries to jot down facts he has learned on a case. Gable had retired from Scotland Yard as a Detective Sergeant before being recruited by Swan to the SID.
Gable has considerable respect for Swan, still calling him 'sir' after all the time they have known each other despite Swan's constant chiding about the formality. In the first recorded adventure, Gable says of Swan that he "is very thorough in all of his investigations. He wasn't nicknamed The Weasel of MI5 for nothing".
The activities described in the books take place over a period of years. The first clearly happens in or around 1964 as the "new James Bond movie", Goldfinger, is mentioned. The second takes place 3-4 years later based on a comment about Gable and how long he has worked with Swan. The third transpires in 1974 when the Turkish invasion of Cyprus occurs.