Ji-hun, a former member of a secret special forces unit under the Special Operations Planning Office of the South Korean Army Headquarters, returns from a long overseas deployment— only to receive the news that his younger sister Ji-yun has died, and that the person accused of killing her is none other than her boyfriend, Seo Min-jun. While still struggling to accept the fact that his sister is gone, Ji-hun, who trusted Min-jun, learns that Min-jun was falsely accused and had gone to Yeokseon Prison in search of Ji-yun’s real killer. But eventually, Min-jun also meets his death inside the prison. Determined to uncover the truth behind the unjust deaths of both Ji-yun and Min-jun, Ji-hun becomes a prison guard and heads to Yeokseon Prison to uncover the secrets hidden within.