Zhang (Shawn Yue) is a policeman who releases snakes in Hong Kong's Sham Water District. As a low-level policeman, Zhang deals with a world that rarely sees the light all day long. It is full of violence and drugs and is a dark corner of society. To survive here, Zhang not only needs to learn how to be a policeman, but also needs to learn the survival rules of the underworld. Gradually, Zhang's identity becomes more and more blurred, and he lives in the gray area between black and white. Lulu (Xian Seli) is a prostitute in this area. Zhang keeps in touch with her, but never moves the relationship forward. In an accident, Zhang accidentally kills the small leader Si Yan Tang (Liao Qizhi). The mysterious killer (Fang Shaohuang) is the only witness. In order to protect himself, Zhang and the mysterious killer start a chase... What secrets are behind the killer? Can Zhang escape this disaster? "Moss" shows us how the little people in the dark of society survive like moss.