Two North Korean soldiers are killed in the Korean Demilitarized Zone at a North Korean border house, before Sergeant Lee Soo-hyeok (Lee Byung-hun), a South Korean soldier on border duties, attempts to flee back to the South Korean side. The southern troops rescue him while gunfire erupts and, two days later, the fragile relationship between the two Koreas depends on a special investigation conducted by Military of Switzerland Major Sophie E. Jean (Lee Young-ae) on behalf of the Neutral Nations Supervisory Commission.