After witnessing his mother's murder at a young age, Kim Ji-yong (Lee Seung-gi) became a detective in order to investigate on his mother's death. He later changed his name to Eun Dae-gu to be unnoticed by his mother's killer. On his way, he reunites with Eo Soo-sun (Go Ara), whom attended the same middle school and gradually develops a romantic relationship. At the same time, he spys on his team leader, Seo Pan-seok (Cha Seung-won), believing that he is related to the killer of his mother's case. As he unravels the truth behind the incident 11 years ago, he learns about his past, identity as well as making deep connections with people around him.