Mainland CID Officer JIANG ZIDAN (Wang Yang), nicknamed “BULLET”, infiltrates a Hong Kong triad group in an attempt to investigate a cross-border drug trafficking case and encounters Hong Kong narcotics inspector WAI CHUN-HIN (Him Law) and female officer CHAN YUET-KI (Cecilia So). The swift and smart BULLET is favoured by the triad leader WU KOON-YAU (Damian Lau), but he is not aware that CHUN-HIN is the trusted one of the triad’s deputy leader HON LONG (Gallen Lo). The kidnapping of WU’s daughter WU KA-LAM (Jolie Zhu) quickly stirs up long-standing conflicts between HON LONG and KOON-YAU and further fuels their antagonism. CHUN-HIN employs police resources to rescue KA-LAM without authorisation and is dismissed, but in secret he still keeps in touch with narcotics officer SO CHING (Toby Leung). HON LONG, trying to arrogate all the power, does all he can, even in collaboration with outsiders, to get rid of any opposition or possible threat. On the surface, he works closely with BULLET and CHUN-HIN, but what lies underneath is indeed immeasurable power struggle, causing endless rivalry between the good and the evil, among these “frenemies”…