The history of divided Korea gave birth to a very unique group of people who were called non-converted long-term prisoners. These one hundred-some people spent about thirty or forty years in the prison and were released only after Korea initiate the democratization process. These communists to the core were repatriated to north Korea with the result of the inter-Korea summit of 2000. We are rather familiar with how they survived the notorious ‘compulsory ideological conversion operation under anti-communist regimes, but we do not know what they did before they were arrested. And it has been totally unknown how many people were executed by the National Security Law. This study focused on two groups of north Korean agents: one is the non-converted long-term prisoners and the other is who were executed by the south Korean authorities. The both sides of divided Korea sent thousands of armed and unarmed agents to each other’s territory. The south Korean authorities announced that the number of north Korean spies that they arrested or killed after 1961 is about 3,000. After the military coup of 1961 led by Park Chung Hee, about 182 were put to death under the charge of violation of the National Security Law. At least two thirds of them were allegedly known as north Korean agents. Like invisible men, it was very difficult to find records on them. What most victims of capital punishment and all the long-term prisoners did in south Korea were these things; spending one or two days at home in the south; meeting with relatives or friends; or merely crossing the border. The Japanese authorities in colonial Korea arrested a lot of freedom fighters, but they were not sentenced to death under these causes. The espionage and counter-espionage efforts implemented by both sides became quite irrational, high-cost, and ineffective. The north Korean authorities find them useless and stopped dispatching agents to the south in 1972. However, the enlarged counter-espionage authorities in the south started forging spy-ring case, victimizing Koreans in Japan or returnee fishermen who had been kidnapped by north Koreans. The espionage and counter-espionage efforts by both sides still left long and dark shadows in each society.