The storyline deals with Meyers' origins, and how the Thorn symbol (on his wrist) signifies he is a curse. There is a subplot involving Jamie from part 5 and cultists, but none of it ever comes together. The ending is takced on as well, as Pleasance died before the reshoot (thankfully they changed it the conclusion from that in the Producer's Cut, where Loomis becomes Meyers' Keeper!). Definately worth a watch.
The bottom line: The most violent film in the series. Halloween 6 is a well edited, dark, atmospheric sequel. True, it never makes sense, but at least it tries to have a plot (unlike Friday the 13th 7). Enjoy, you cheese lovers!