綽號“校長”的江湖老大被自己的手下阿一殺害，繼室「小嫂」也被阿一占有。只要擁有校長代表“龍頭”地位的權杖，阿一就可以名正言順的成為黑幫老大，但校長在臨終前已把權杖交給了孫女小雨，阿一只好向小雨下手。負責保護小雨的警探阿飛及小雨被追殺之際，幸得校長的故交神槍手與智多星及時趕到，把他們救走。為了履行多年前對校長的承諾，神槍手與智多星重出江湖保護小雨，同時也要幫校長報仇。于是，以阿一為首的黑幫、警方、神槍手與智多星，圍繞著小雨和權杖展開了一場鬥智鬥勇的正邪之戰，過程奇峰突起，結局詭異難料！

This was a legend told and re-told in a sinful city... In the midst of the gunfights and bloodshed in a sinful city, there were two legendary figures - known only as 'Bullet' and 'Brain'. 'Bullet', as the name suggests, earned a reputation for being the sharpest shooter the city had ever seen. He could take out his enemy with a pistol in less than one tenth of a second. 'Brain', on the other hand, earned his reputation for his ingenious plots and the countless electronic gadgets he was equipped with. Unknown to all, both Bullet and Brain have separate incidences where they swore never to kill and manipulate people. Soon they disappeared and no one has heard from them since. Nine years passed quickly, and the stories of Bullet and Brain have become only tales of legend. However an old debt and promise draws Bullet and Brain back to the city but this may be their last mission...