Cheung Wai-kit (Nicholas Tse) and Ngai Fung "Fit Lo" (Edison Chen) are best friends and two of Hong Kong's top young cops. After foiling a robbery attempt they are promoted to the Narcotic units led by hot-headed police chief Cheung Tit-Man (Simon Yam). Tit-Man is actually Kit's dad, who left him and his mother (Michelle Mai) after shooting her years ago. Kit sees Tit-Man as an enemy and is not so thrilled with having to work under him. Kit requests for an transfer to the internal affairs department, but decides to participate in one last operation where the team tries to take down a major drug lord known as Big Bryan (Ken Tong). During the arrest attempt, Fit Lo is seen letting Bryan go, which costs him his job. Now seemingly on opposite sides of the law, Kit and Fit Lo must try and keep their friendship together. To make things even more complicated Fit Lo is also married to Kit's former love interest and cousin Wing Chi (Gillian Chung).