Kim Taek Rok is a veteran detective at the Geumo Police Station waiting for his retirement. One day, he starts receiving phone calls from someone who claims to be an old friend. The caller tells him that he killed Kim Taek Rok’s colleague Woo Hyun Seok, suggesting looking into Kim Taek Rok's past to find things he did wrong. Initially, Taek Rok's Investigation Squad Chief Kook Jin Han, who recently transferred to Geumo Police Station, suspects Kim Taek Rok as Woo Hyun Seok's murderer, but he begins to trust Kim Taek Rok and his innocence. They begin to look for Kim Taek Rok’s mysterious caller with the help of detectives Lee Sung A and Son Kyung Chan. (Source: AsianWiki; edited by MyDramaList)