With Korea's crime rate on the rise in Seoul, the Seoul Police are doing everything they can to keep their citizens and themselves safe. Chief Lee Dongwook is at the prime of his career with strong, intelluctual individuals backing him up. Senior Inspector Choi with his wise and fair judgement, looks after the young team members and handsome Inspector Minhyun with his fast wits, technology savvy and capability.

The Seoul Police Second Bureau has one special member on their team. Zero. A hardworking, highly skilled and trained individual in the line of street work. She has the most arrest rates and helped solve a majority of cold cases. She is highly praised within the police community for her work and feared by many underground despite not knowing her true identity Although she is in partnership with the police, she is on neutral sides. She only works for her own personal gain and by that means, her reputation has to be known within both sides.

Yoon Doojoon is a passionate new inspector that joins Chief Lee's team. Ranked #1 after graduating the Police Academy, he strives to be a respected inspector. He goes by the book of conduct and has a strong sense of justice. Once he joins the team, he has to quickly adapt to the use of Zero and prove himself to be skilled, but unfortunately, with the series of upcoming cases, mishaps happen. 

New cases then bring together two individuals who are of two different worlds. Quarrels happen, fights erupt and possibly... Love?