Confession of Pain

A man named Yau Kin-bong works as a police investigator with his partner Lau Ching-hei. One day when Yau comes home he finds his girlfriend dead from slitting her wrist. Yau is devastated and decides that he wants to investigate to figure out why she would commit suicide. When he starts investigating things start to unravel. What does Yau find out during his investigation?