There is a place (The Village) where people are taken if they have information others want or others want to be kept hidden. The Village is part prison camp, part holiday camp, and part psychiatric institution. The inmates do not know which government runs The Village; perhaps the Soviet Union, perhaps the West. They have sophisticated surveillance, crowd-control and interrogation techniques, some of them experimental. They also seem to have secret technology not allowed in the outside world.
The people who run The Village are determined to extract information from the hero who has been abducted and taken there. We know him as "number 6". Everyone there has a number not a name. The person who appears to run The Village is called "number 2" and several different people in turn have that position. They all seem to be upper-class Englishmen, but this is no evidence as to which government runs The Village.
They want to know more than anything why he resigned. They seem to be using the milder forms of interrogation and mind-control to begin with so as not to destroy his sanity. However, as time goes by and they don't get results they use increasingly harsher methods.
The series had a wonderful theme tune and title sequence.