Basically a kids movie, so you need to watch it in that context to appreciate it, I think.
A group of teenage girls go camping, but have to take shelter in a temple when it rains. One of the girls (Bonnie Law) takes a piece of rope from the temple, little realising that it contains the spirit of ghost Raymond Wong, trapped there for many centuries.
Back at Bonnie's house, Raymond manifests himself and asks Bonnie to return the rope. Bonnie is pretty freaked out by the whole ghost thing, but eventually Raymond wins her over. From there, Raymond proves what a friendly ghost he is by helping the girls out in a variety of situations - most prominently and hilariously helping Bonnie against a girl from a rival school in a sports competition.
Raymond becomes their friend, and they all have a lot of fun together... but eventually the situation becomes more serious. Not everybody thinks having a ghost around is a good plan, and soon it's exorcism time - a situation that is going to test their friendships.
The movie contains a lot of funny moments, with Raymond Wong being extremely charismatic in the titular role. The movie is not afraid to address a couple of more serious issues, though. You can see it's trying to get a message or two across to teenage girls, and it manages to do so without being too patronising.
More of a curio than a must see, but a pleasant diversion. My only complaint would be that Bonnie Law gets much more screen time than Loletta Lee, although given the 'jailbait' aspect of Loletta maybe it's safest that way :)