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”…

During the 1944 Wang Jingwei regime of the 2nd Sino-Japanese War, Lu Feng, an orphan, returns home to Shanghai after studying for two years in Japan. Unbeknownst to the world, he has been ordered to enter the No. 76 Headquarters as an undercover agent for the Communist Party. Along the way, Lu Feng encounters the mysterious Hua Xiang Yu, a capable and skilled woman who has just returned from studying abroad in America, and his old childhood friend, Huo Fei. Both individuals seem to hold secrets of their own. Time and time again, as Lu Feng strives to protect his undercover identity, he finds himself on the opposite side of Hua Xiang Yu. Slowly and uncertainly, the two circle around each other, each hoping to uncover the other's real purpose. In this time of chaos where corruption and violence dominate the world, these underground agents, can only choose to fight for what they believe is right... and, hopefully, bring about a more peaceful tomorrow.