Yoon Ji-woo's (Han So-hee) father dies suddenly. She desperately wants to take revenge on whoever is responsible for her father's death. Ji-woo then works for drug crime gang Dongcheon, led by Choi Mu-jin (Park Hee-soon). With the help of Mu-jin and to uncover the reason for her father's death, Ji-woo joins the police force and becomes a mole for Dongcheon. Ji-woo, using her fake name Oh Hye-jin, is assigned to work in the drug investigation unit in the Inchang Metropolitan Police Station. Her partner there is Detective Jeon Pil-do (Ahn Bo-hyun).
