During the end of Korean War, despite ceasefire negotiations, the fighting continues around the Aerok Hills on the eastern front, as that would help determine the future dividing line between the North and the South. The hills continue to change hands, so that even the ceasefire negotiators don't always know who controls them.

Amidst the fighting, the South Korean officer commanding Alligator Company, who are fighting for Aerok Hill, is found dead, by a Southern bullet. First Lieutenant Kang Eun-Pyo of the South Korean Army's Counterintelligence Corps (the precursor to South Korea's current Defense Security Command) is sent to investigate the murder and also an apparent mole who had mailed a letter from a Northern soldier to his family in the South.