Je-0 Gyodogwan

Ji-Hun, a former member of the secret special forces under the Special Operations Planning Division of the Republic of Korea Army, returns from a long deployment and learns of his younger sister Ji-Yun's death and that her boyfriend, Seo Min-Jun, is the killer. Ji-Hun, who struggles to accept his sister's death, trusts Min-Jun and learns that he has been falsely accused and sent to Yeokseon Prison to find Ji-Yun's killer. However, Min-Jun also dies in prison...

Ji-Hun becomes a prison guard and heads to Yeokseon Prison to uncover the secret behind the two men's unjust deaths.

(Source: Naver, translated)