經營非法紅油的黑道油王霍青松(陳奕迅 飾)因一次油車爆炸意外成為香港警方及國內公安眼中釘，此時要面對獨生子被綁架的他顯得心焦如焚。
高級督察司徒慕蓮(鄭秀文 飾)是警隊明日之星，以精明幹練見稱警界，她接手偵查霍青松之子綁架案的任務，帶總動員進駐霍家，逼他與警方合作救出兒子。
綁架案動員全港警員以至內地公安。當中包括有份參與圍捕紅油集團且目睹油車爆炸的公安隊長趙天河(董勇 飾)，他與意外中死者的丈夫徐半山(張國立 飾)結成知交，卻對青松恨之入骨。 黑白兩道破天荒攜手合作，雙方卻衝突連連，偵查行動寸步難移，慕蓮發現自己懷了身孕，陷於情感危機；青松救子心切又要保住幫派，左右做人難。經過種種難關，慕蓮與青松最終會否化敵為友﹖綁匪幕後主腦又到底是誰﹖真相耐人尋味，案中有案之離奇綁架，墜入曲折離奇之大搜查..

John Fok (EASON CHAN) is the kingpin of illegal red oil in Greater China. When an oil tanker explodes accidentally, he becomes the focus of investigation by Hong Kong and Mainland Chinese police and is forced to halt his business to wait things out. While his business rivals start closing in, a member of his household is planning to betray him. On the surface, John seems totally helpless, but in reality, he’s been staging a major comeback for months. Just as he is finally ready to turn his fortune around, however, his only son is kidnapped!

Senior Inspector Molline Szeto (SAMMI CHENG) is the rising star in the police force, who is well praised for busting crime with her characteristic sangfroid and precision. However, in matters of love, she is totally at wit’s end. After dating the same man for more than ten years with no likely prospect of an imminent wedding, she determines to cut the knot once and for all, before she becomes too old for childbirth. Poised at the crossroad of her life, she is thrilled to be assigned to investigate the kidnap of John’s son.

John dispatches his whole gang to ferret out his son’s whereabouts, only to find out that his rivals are redeploying people in China to assail him. Just when he is about to order an all-out fightback, Molline suddenly arrives with her team to garrison at his house. Spurning her at first, he soon finds her critical analysis of the situation at hand both apt and useful. An unlikely cooperation between cops and crooks thus ensues, leading to conflicts and clashes galore at every step till the very end.