Mike Stone is an agent with the British Secret Service.
More specifically, he is attached to the Special Security Department of that agency. How that sub-agency differs from the overall organization is never mentioned, just that it is special in the jobs it is given.
The year of his activities is 1972 and Britain and the West are very much involved in competition with the agents from the "other side" although no actual names are mentioned. While there will also be a couple of adventures outside the border of England, most of the activities that occupy Stone in the handful of recorded adventures take place inside the U.K.
His immediate supervisor at the department is a white-haired aged man with a thick mustache and an obvious liking of Stone. The two men maintain a professional but very friendly relationship.
Stone is a tall, slender man likely in his early 30's with black hair worn fairly short for that time period. By and large he comes across to all who work with him as a pretty easy going fellow, quick with a grin or a quip and polite to all. His opponents do not find him quite so friendly, especially if they start the conversation by trying to kill him; he finds that objectionable and goes out of his way to show them that.
We never learn anything about Stone outside his work. Obviously he has a personal life but he does not show any signs of letting it take any role in his professional one. Or he just lives for his exciting career, which would explain why he seems to happy and eager to leap into any mission.
Since most of the missions we have of Stone require a fair amount of investigation, either he is quite good at it or he is very proficient at waving a flag at his opponents daring them to come get him. Considering how many of them actually do just that, the latter seems a safe bet.