Prison Guard Zero

Jihun, once part of an elite secret special forces unit, returns from overseas duty to the devastating news of his sister Jiyun's death. Her boyfriend Minjun is blamed, but Jihun soon learns Minjun entered Yeokseon Prison under a false charge to uncover the real killer. When Minjun is found dead inside the prison, Jihun refuses to accept the official story. Taking on the role of a correctional officer, he infiltrates the prison to expose the truth behind both murders—and the hidden forces controlling the facility.

(Source: Webtoon Entertainment)