Dark Figure of Crime (2018)

Hyung Min is a respected detective. He is aware of Tae Oh who is in prison for murder. One day, Hyung Min receives a phone call from Tae Oh in prison. Tae Oh confesses to Hyung Min that he committed 7 more murders and gives Hyung Min clues on the murders. No one believes Tae Oh's confession except for Hyung Min. Without knowing ab who the victims are, Hyung Min attempts to uncover the 7 murders.