来自柬埔寨的杀手鹏(陈冠希饰)，吃垃圾长大，个性好勇斗狠，接下一宗刺杀法官太太的单子，来到香港。藏身垃圾堆的他，无意中发现一个隐身垃圾屋的女孩，智障，被父强奸。出生于香港的重案组探员伟(李璨琛饰)，行事偏激，追踪法官太太谋杀案，在与鹏的交锋中，全部同事殉职。二人互相追捕互相角力，以暴制暴，狗咬狗般的追逐和厮杀开始……

    ゛  ▄m  ▄＊  
      ┠ ┨┠ ┨         

a banjo on my knee，
I’m going to Louisiana
my true love for to see.
It rained all night the day I left，
the weather it was dry
The sun so hot， I froze to death
Suzanna don’t you cry.
Oh Suzanna， oh don’t you cry for me，
I’ve come from Alabama with
a banjo on my knee.
I come from Alabama with
a banjo on my knee，
I’m going to Louisiana
my true love for to see.
It rained all night the day I left，
the weather it was dry
The sun so hot， I froze to death
Suzanna don’t you cry.
Oh Suzanna， oh don’t you cry for me，
I’ve come from Alabama with
a banjo on my knee.