"It's a police dog, a dog that doesn't catch criminals but policemen!" Jiang Yingjun, the top student among the police academy graduates, is a cold-blooded policeman who doesn't even care about the smallest favor. Yingjun works in the Internal Affairs Department, and the police call him "police dog". As long as he takes over a case, no matter who the other party is, he will dig out the truth through persistent investigation. Yingjun discovered that his colleague's corruption case was a big case in which a policeman was involved in a drug trafficking group, so he went to Busan, where no one wanted to go, to start a joint search. "That's why he is called the 'dung fly detective'!" Jiang Minhao, the leader of the customs group, is very humane. With his experience as a team leader for thousands of years, he is good at handling cases based on intuition and is known as the "dung fly detective". However, his years of experience and his considerate attitude towards his colleagues' families have earned him the respect of his young colleagues. The Seoul Headquarters suddenly asked to send someone to participate in the joint search, and his son Yingjun, who he hadn't dealt with for 8 years, was the person in charge of the case! Officer Jiang Yingjun VS Team Leader Jiang Minhao, a new partner is born! Unlike Yingjun, Squad Leader Jiang was still very excited to see his son again, but he always felt awkward with Yingjun. But no matter what, now the two are partners in solving the same case! However, their styles and working methods have nothing in common, and their relationship is as tense as a time bomb ready to explode. The search has been ineffective, and finally, Team Leader Jiang's intuition flashes with light, and there is a major clue to the case. The two partners then begin to cooperate sincerely and truly act like partners. However, this state is short-lived, and the unexpected inside story of the police is revealed bit by bit...