Mark Nicholson is an agent with Counter-Terror.
This organization is dedicated to fighting terrorism in Europe. The late '60s and early '70s had seen a dramatic rise in attacks by a wide assortment of groups, each with its own list of grievances and desires. Leaders among these were the IRA, Basque separatists, Baidor-Meinhof, Red Army, PLO, and so on. The attacks were growing bolder and more frequent.
Realizing that there was no central group able to handle the crisis because Interpol was constitutionally prohibited from dealing with political crimes, the nations of Europe created a small group to fight back. Under the leadership of Sir Alexander Gwynne-Vaughn, the team was comprised of leading police and anti-terror experts from major law enforcement groups seconded to this new agency.
The person put at the front of the field operatives, the man heading most missions, is Detective Inspector Nicholson, later promoted to Detective Chief Superintendent. In his mid-30's when the series begins, he is transferred to Counter-Terror from his former job at Special Branch though in his new job, he continues to have an office in New Scotland Yard and has the benefit of all his normal contacts and friendships, both of which he will make intelligent use of to accomplish his missions.
He is described as slender with dark hair and a rugged, sturdy jaw-line that easily conveys strength and determination. These traits are very useful for getting the information that sets up his missions but it is dogged attention to detail and the ability to operate alone and under pressure that keeps Nicholson alive out in the field.