ZHANG Zi-ming shot his childhood friend CHEN Jin-huo dead while on the run. CHEN Jin-fa, Jin-huo’s brother and local detective, swears to bring ZHANG to justice as he struggles to accept how ZHANG can be so coldblooded. Panic and shock surge the society after the gunfight. Under public pressure to close the case, the government sends LI Wan-ji, an evidence-based detective with doctorate in criminal psychology and behavioral science, to cooperate with CHEN Jin-fa. Their contrast in experiences and personalities brew conflict during the investigation. However, they have to work together to face their mutual enemy, a cunning gunman with many weapons. At the same time, friendship grows through their constant disagreement with one another and fighting alongside each other.