 After witnessing his mother's murder at a young age, Kim Ji-yong (Lee Seung-gi) became a detective in order to investigate 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), who attended the same middle school and they gradually develop a romantic relationship.