Roy Rickman is an agent for the PIC.
Patterned after a similar legislative structure in America, the relatively new Parliamentary Investigatory Committee (PIC) is empowered to look into anything affecting the security of the country, all as a research tool to give Parliament more information from which to govern.
At the beginning of the series, Rickman had no connection to the PIC. He was a junior administrative worker in Personnel for a mid-size corporation, doing a fairly boring job. His only excitement in life was the affair he was having with a married woman. Then he learned that the Intelligence community had prepared a list of people like him and from that list as needed they would pick a name and, with that person's permission, have an agent assume that identity. It made it far easier to have a convincing back-story when the identity being used was a real one.
He agreed, mostly as a lark. Then the agent using his name is murdered. End of lark.
Rickman is told that the agent who died had worked for the PIC and the head of that branch wanted Rickman to take his place - in essence pretend to be the man pretending to be himself. From then on, Rickman is a full-time agent in the department.
This series presents an extreme view of the political scene at the time of its writing. Socialism was rising fast and bordering on communism with pressure from inside and outside to leave the Western alliances and join the Soviet bloc. The in-fighting is tremendous and the pallor is bleak. Whether either side is better or worse is something that Rickman never bothers to decide as he is too busy just trying to stay alive.