After numerous setbacks, the capable, courageous, beautiful and optimistic Yu Xiushan (played by Gao Yala) finally passed the police civil service examination and became a criminal police officer at the Gangnam Police Department as she wished. She and Yin Daegu (played by Lee Seung-gi) who had a deep hatred, Park Tae-il (played by Ahn Jae-hyun) who exuded aristocratic temperament, and Ji Guo (played by Park Jung-min) who was sneaky and unambitious were assigned to the powerful 3rd team led by Team Leader Xu Panxi (played by Cha Seung-won). Team Leader Xu is brave and heroic, and is an excellent criminal police officer who has made great achievements. However, these four rookies with different personalities and constant troubles are really difficult. Not only are the four people in constant situations, but they also disrupt the actions of other departments from time to time. Even so, Team Leader Xu is determined to train them well in three months. Along with the arduous training, various cases occur from time to time, and the things that Daegu and Xiushan themselves are pursuing gradually surface...