In drug crime circles, 'Ya-Dang' informants sell criminals' info. Criminals use this to reduce sentences, while law enforcement uses it for arrests. Ya-Dang, police and prosecutors form a key triangle.

Hwang Ki Joon, Mr Action, and Kang Hee Yeol, Mr Bookworm, are two best friends but contradictory students at Korean National Police University. They accidentally witness a kidnapping and decide to work together to track it down.