Detective Hei (Leung), a kind-hearted veteran police officer and his partner Bong (Kaneshiro) bust a rapist in 2003. When Bong returns home after the bust, he finds his pregnant girlfriend has committed suicide. Since then, Bong becomes an alcoholic and turns in his badge to work as a private investigator. He is obsessed with finding out why his girlfriend committed suicide years ago.
Three years later, Bong discovers on the night of his girlfriend's suicide, she had been waiting at a bar for her secret lover, who wound up unconscious after a car accident the same night. Bong originally thinks that he will be angry at his girlfriend's lover, but he ends up taking care of the comatose lover while he lays in hospital. Bong eventually moves on and falls in love with a girl who sells beer in the same bar, Feng (Shu Qi).
Hei marries Susan (Xu Jinlei), daughter of billionaire Chau. Hei and two accomplices (two lowlifes) break into Chau's home to rob and kill Chau and his butler, Man. Later Hei kills his accomplices making it look like they killed each other in a dispute over the loot. Susan isn't convinced about the two killers being alone, so she hires Hei's friend Bong to investigate further. Bong discovers Hei murdered Chau for revenge. Chau had killed Hei's family when he was a boy living in Macau. Hei then took on a new identity. Hei then tries to kill his wife Susan to end the Chau family line, but she survives the attempt and goes into a coma.
When he visits Susan in hospital, he mentally confesses but all along Susan knows the truth and asks him if he ever loved her. Hei then realizes that in his desire for vengeance for his family, he destroyed the new family that he created with Susan. Susan doesn't believe him and loses her will to live. Unable to bear the pain and anguish, he shoots himself in the head in front of his wife's dead body in the hospital.