Within this movie, Shaggy and Scooby, who are innocently trick or treating, end up traveling to Halloween Land in order to save the world from eternal Halloween and to stop an evil magician who's trying to obtain ultimate power by conning the Goblin King out of his scepter, which he will use to get his revenge on them. The plot itself is pretty complex, especially for a Scooby-Doo movie. About this time, though, the Scooby-DooDirect-to-Video movies started taking a turn for the better when movies with improved animation and funny moments started coming out, like Scooby-Doo! Abracadabra-Doo. Still, this is one of the movies where Shaggy and Scooby are featured as the main protagonists.
And how do they stop said magician? They get the help of all the people that they encountered during the trip. Props especially go to Jack who returns despite the fact that his candle burns out by sunrise.