[1][2] 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.

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.

They become partners again, to catch the Subway Ghost together with their team, and to search for Yoo Ryeong's sister.