Laughing（謝天華）本是警方的臥底，但因為一宗販毒案件，被警司潘SIR（黃日華）追捕。

其實Laughing加入警隊之前，是個黑幫成員，一直跟隨其老大一哥（黃秋生）在道上打混。一哥對Laughing照顧有加，Laughing視一哥有如親兄。一哥為了保護自己的地盤及非法生意，竟要求Laughing投考警隊，讓他可以掌握警方行動。

Laughing在警校成績優異，可惜天意弄人，他根本沒機會待在警隊，因為反黑警司冼SIR（元彪）被Laughing的黑道氣味所吸引，竟指派他做臥底探員。

就這樣，Laughing竟成了「雙重臥底」，一哥也樂得Laughing被送進社團去對付自己的同門座頭（吳鎮宇）。在這已經複雜的狀況，一件事情令Laughing的處境更複雜，他竟愛上了座頭的妹妹Karen（陳法拉）。

面對法治、恩義、感情的抉擇，Laughing將如何自處？無論選擇哪一邊，都是一場變節，Laughing最後將投向哪一方？

Laughing (Michael Tse) is formally a police undercover. However due to a drug trafficking case, Officer Pun (Felix Wong) is hot on his trails.

Before Laughing becomes a police undercover, he works under triad Brother One (Anthony Wong). Brother One treats Laughing as his own brother, however to protect his territory and illegal businesses, Brother One instructed Laughing to join the police force and act as his undercover.

Laughing is a high-flyer in the police academy. However he never got a chance to join the force because he is specially picked by Officer Xian (Yuen Biao) to be an undercover in the underworld syndicate. Hence Laughing becomes a “double-undercover”.

Brother One is happy that Laughing is allocated to the triad of his rival (Francis Ng). To make his situation worse, Laughing falls in love with the triad leader’s sister, Karen (Fala Chen).

Justice, brotherhood and love, what will Laughing decide on? His decision will be a turning point in his life.