Dan Mitchell is an agent with the British Deep Cover Agency (DCA).
That intelligence organization is said to be a part of the Foreign Office, at least it is located in the British Cabinet department's headquarters. It is run by a fellow named Harold Mellish who "rarely saw field operatives" though he was the one "responsible for enticing me into his team of faceless wonders, almost on Graduation Day at Cambridge".
Mitchell has been with that organization for roughly three years when we meet him in the first recorded adventure. He tells us that he had been recruited by DCA in part because of his proficiency in the Russian language "including a few dialects". He had taken up study of the language while in college and his studies had included a year living just outside St. Petersburg (as it is now known). As a result he could write and speak it as if a native.
But having achieved his goal, he decided he was not interested in further use of it so when the DCA came knocking with an offer, he turned it down, thinking it was likely for a desk job which did not interest him. Instead, he "had this misguided yearning for a more physical life and took a commission at Sandhurst before realizing [he] wasn't really officer material". He chose instead "life as a squaddy in the Parachute Regiment" followed eventually by his real ambition of getting into the SAS.
"It wasn't surprising however, that when a combatant is slung out of such an institution as the SAS for being over indulgent in the physical art of fighting, he was bound to catch the attention of the Security Forces." That included the DCA and this time he accepted.
When we meet him, Mitchell had just returned from the Soviet Union (the activity taking place likely in the late 70s or early 80s) where he had managed, under DCA orders, to infiltrate the "elite regiment known as the Spetsnaz". This return was because, following his orders, he had been engaged with and taken out half a Spetsnaz unit which made Moscow "peeved to say the least".
-"I still wasn't sure whether what I'd been told was genuinely the whole story, part of the story, or just a load of fiction shared to justify my deployment."