Kit (Nicholas Tse) and Fit (Edison Chen) have been friends since the Police Training School. However, twists of fate have led them along different roads. While working on an assignment, Kit by chance meets his father, Cheung Tit Man (Simon Yam), whom he has not seen for 17 years and he regarded as his enemy.
Later on, Kit and Fit are transferred to the CID which is led by Cheung. Their mission is to destroy the Mongkok gang leaders, the Bryan brothers. At the same time, the Bryan brothers set a trap to try and force Fit to join their gang. Meanwhile, Kit's increasingly neglected girlfriend, Wing (Gillian Chung), falls for Fit and is bearing his child.