2019年，珠寶劫案悍匪陳祖耀的冷血一槍，改寫四位機動部隊警員的命途。五年後，四警當中，只剩關智軒留守機動部隊；徐國賢退出警隊，更加入了黑幫……智軒偕拍檔蘇國威領着機動部隊新組員，協助反黑組督察彭穎琪抓捕毒販，竟再次遇上祖耀！祖耀一心要向人稱「西九王」的毒梟無良報仇，其後祖耀成功復仇，卻沒有令西九變得太平。各方惡勢力崛起，西九反黑組和機動部隊為維護社會安寧，與黑幫周旋，激戰連場，無日無之！
In 2019, the cold-blooded shot of jewelry robbery robber Chen Zuyao changed the lives of four mobile police officers. Five years later, among the four policemen, only Guan Zhixuan remained in the mobile unit; Xu Guoxian quit the police force and joined the gang... Zhixuan and his partner Su Guowei led the new members of the mobile unit to assist the anti-gangster inspector Peng Yingqi in arresting drug dealers. Zu Yao was bent on revenge against the unscrupulous drug lord known as the "King of West Kowloon". Later, Zu Yao succeeded in revenge, but it did not bring peace to West Kowloon. Evil forces from all sides are on the rise. In order to maintain social peace, the West Kowloon anti-gangster group and mobile units deal with gangsters and engage in fierce battles every day!
警钟大鸣，警察关志轩、徐国贤、树哥和灿哥追捕刧匪，冷血悍匪抢去警枪，当场击毙树哥！亡命的匪徒，冷血的一枪，改写了四个警察的命途…… 树哥殉职，关志轩再次加入机动部队，灿哥被调至警察杂项调查队，徐国贤退出警队，混入西九龙最大的黑帮，辗转做了西九王无良的头马小贤哥，过着刀光剑影，杀声震天的非人生活，底牌却是警察潜伏黑社会的卧底。 西九王无良死后，帝位悬空，群雄争夺，又来一场血雨腥风，死伤无数。戴警司要求徐国贤继续当卧底，且顺水推舟接管无良整个社团势力。岂料，徐国贤卧底身份败露，黑社会对头人陈祖耀派人追杀徐国贤，徐国贤差点命丧黄泉，身负重伤。 人生交叉点，徐国贤的抉择，黑社会每年数十亿毒品交易的西九王，还是做回警察，当一个月薪只得三万多的反黑英雄？
