Hong Kong customs officers discover illegal weapons on a cargo ship and trace them back to an international conflict. Elite agents go undercover, uncovering a transnational operation with roots closer to home.

Hong Kong Customs and Excise Department officers Chow Ching-lai (Nicholas TSE) and Cheung Wan-nam (Jacky CHEUNG) share a close mentor-student relationship. On a case involving seized smuggled arms, Ching-lai is sent to take part in a joint operation with Interpol to capture a reclusive arms dealer. However, Wan-nam makes a drastic decision that complicates the case even further.