Yoo Ryeong became a police investigator in order to find her missing twin sister. She was disappointed because the report of losing her sister was ignored by Ha Ma-ri, the police officer at that time. She thinks her sister was a victim of a serial murder by the Subway Ghost, and she does not always follow the rules as she strongly believes in justice. She partners with Go Ji-seok. On the night of her shift, with the help of Kim Woo-hyuk (a police officer at metro who is interested in her perseverance), she searches for her sister in a tunnel without her boss knowing. Meanwhile, Go Ji-seok became famous because of his looks, and for being the best police officer in his class. He gets a promotion to be a metro police officer with his then girlfriend Ha Ma-ri, but circumstances force him to be a police officer at the subway, and that leads to his breakup with her. Unlike Yoo Ryeong, he strictly follows the rules and doesn't like trouble. His partnership with her thus leads to many problems for him. However, due to Yoo Ryeong's sincerity for every case, he begins to think about his past. Later, Yoo Ryeong wants to separate from her partner because she doesn't want to always depend on him. Ji-seok is devastated, but he understands his partner's decision. Inspector Gong, Yoo Ryeong's new partner, is frustrated with her antics. Yoo Ryeong and Ji-seok work together to solve a kidnapping incident at the subway. After their collaboration, the two realize they miss each other. Knowing that Inspector Gong can no longer handle Yoo Ryeong, Ji-seok makes a deal to take back Yoo Ryeong as his partner without her knowing. They become partners again, to catch the Subway Ghost together with their team, and to search for Yoo Ryeong's sister.
