In the wake of the Blue House Raid (where North Korean special forces infiltrated the DMZ just to kill South Korean President Park Chung-hee at home), the South Korean President launched a plan of his own. He ordered the Korean Central Intelligence Agency (KCIA) to plan a retaliation. The KCIA conscripted 31 petty criminals and unemployed youth to train for a singular purpose: to assassinate North Korea's dictator Kim Il-Sung.
They formed Unit 684 on the uninhabited island of Silmido in the Yellow Sea off of South Korea's West coast. The training was so brutal, seven members did not survive. Unfortunately for the members of the 684, a thaw in relations occurred before their mission was launched. The entire mission was shut down.
In August 1971, members of Unit 684 inexplicably overpowered their guards, killing all but six, and made their way to the mainland. Once there, they hijacked a bus to Seoul but were stopped by the Army. Twenty members of the unit were shot or committed suicide with hand grenades. The survivors were tried and executed.
The South Korean government covered up any information regarding Unit 684 until the 1990s. They refused to divulge any information about the events even after a 2003 movie was released. South Korea did not release its files on 684 until 2006.