Olivia Cheng plays a social worker married to a police photographer. They have one son upon whom the husband (Eddie Chan) dotes on to an almost unnatural degree whilst she is busy saving the lives of the teenage tearaways in her care...
The streets of Hong Kong never looked so dark (literally - there looks like there was not much budget for lighting so often its hard to see what's going on - and morally). Rape, arson, prostitution, violence, drug taking... seem endemic on the mean streets of Kowloon!
A tragic string of events that starts off with a brutal rape leads to the death of Eddie's and Olivia's son and then Eddie goes - well crazy. He starts to kill off his wife's clients using various methods such as apartment window assisted projection.
As the film continues he becomes more and more insane, in the end wanting to kill himself and Olivia as he thinks his dead son is lonely and is waiting for them. The ending is a bit unsurprising though rather gore splatter-tastic.
A recommended little horror gem from the early 1980s. Don't expect a barrel of laughs though.