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.
