On 31 January 1968, 31 commandos from North Korean Special Unit 124 infiltrated South Korea in a failed mission to assassinate President Park Chung-hee.
As a means of retaliation, the South Korean military assembled a team of 31 social outcasts and death/life criminals, training them on the island of Silmido, off the coast of Incheon, in order to execute Kim Il-sung. The mission, regarded a suicide mission, was seen as the only way for these new recruits to redeem themselves and show their loyalty to their country. If they succeeded, they would win their freedom and a new life. With this goal in mind, they endured gruelling, almost inhumane training, becoming finely honed killing machines.
In October 1968, at the end of their training, they were dispatched on their mission to the North, but were recalled not long after their departure due to mysterious orders from above. The project was called off, and they returned to Silmido discouraged and frustrated. This frustration led two squad members to go AWOL and rape a female doctor who lived relatively nearby; they were discovered, surrounded by military, and realizing their fate, attempted mutual suicide. One man was killed in this, the other injured. The latter was then returned to the camp on Silmido and strung up, alive, as an example while his fellow squad members were beaten by the troops for the two men's betrayal. One member snapped, charging to the front and killing the strung-up man. Meanwhile efforts continued to negotiate peaceful reunification between the two Koreas, rendering the squad all but useless.
To keep the top-secret, dirty project unknown to the outside world, the South Korean intelligence agency decided to "eliminate" the squad. The commander, outraged, is then told that if his troops fail to follow this order, they too will be killed. Torn between his duty to follow orders and his personal honor, the commander intentionally leaked this information. As one high-level officer is sent away from the island for his unwillingness to participate (lied into thinking he's going to re-negotiate the murder order), the squad realizes that they are to be killed that same night, and lays a plan to fight back. They kill their trainers, and then after being told the truth of the mission - that they legally no longer exist, and thus would neither receive recognition for their mission if it succeeded, nor even be allowed to return to society - decide to escape from the island and make their story known. On 23 August 1971, the 20 remaining members of the squad captured a bus and commandeered it for Seoul. An official pronouncement is made through the media that 20 "armed communist agents" had infiltrated the country, and a state of emergency was declared. Eventually they are stopped by army roadblocks and fired on. Realizing their plight, the men proceed to engage in mutual suicide, all throwing their hand grenades into the middle of the bus. The aforementioned high-level officer learns of what is happening and tries to call back the army, but fails. Later, an investigation is carried out and the resulting report filed away, unread.