In 1968, in prisons across South Korea, they were given a choice: fight and win a pardon, or stay in jail. They chose to fight.
Taken to Silmido island, off the east coast, the 31 criminals formed 'Unit 684'. They were put through brutal training by Special Forces soldiers. Their mission was too risky even for the South Korean Army's Headquarters Intelligence Detachment (HID): to infiltrate Pyongyang and take the head of the 'Great Leader', Kim Il-sung.
But politics intervened. The unit had been activated after northern commandos had attempted to kill southern president Park Chung-hee in 1968. When North-South relations improved in 1969, Unit 684 had no role. Members were killed off, one by one, in training accidents.
Wising up, the survivors fought their way on to the mainland, where they commandeered a bus and headed for Seoul. What they planned to do upon arrival can never be known. On August 23, 1971, in a roadside ambush, they were killed by regular troops.
Thus the plot of Silmido (2003), the first South Korean film to attract 10 million local viewers. Its key attraction was that it was based on a true story. 'It brought out something the government had hidden, but that people remembered,' said Silmido's producer, Jonathan Kim of Hanmac Films. 'People had been wondering what had really happened, so the film became a conversation piece.'
In a curious footnote, a South Korean television documentary this year claimed that not all Unit 684 members were killed: it alleged that some are today living in secrecy.