Kim Taek-Rok (Lee Sung-Min) is a veteran detective at the Geumo Police Station. He is waiting for his retirement. Kuk Jin-Han (Jin Goo) gets transferred to Geumo Police Station as the chief of the investigation squad where Kim Taek-Rok works. One day, Kim Taek-Rok receives a phone call. The caller tells Kim Taek-Rok that he is an old friend. That is how the mysterious phone calls his begins. On a call, Kim Taek-Rok's old friend tells him that he killed Kim Taek-Rok’s colleague Woo Hyun-Seok. Kim Taek-Rok soon becomes a suspect in the murder of Woo Hyun-Seok. The caller suggests that they should look into Kim Taek-Rok's past to find things he did wrong. Initially, Kuk Jin-Han suspects Kim Taek-Rok as the murderer in Woo Hyun-Seok's death, but he begins to trust Kim Taek-Rok and his innocence. They begin to look for Kim Taek-Rok’s old friend with the help of detectives Lee Sung-A (Kyung Soo-Jin) and Son Kyung-Chan (Lee Hak-Joo).