Halloween III: Season of the Witch is infamous for its (in retrospect) baffling decision to not feature Michael Myers or even be set in the same universe as the original two films. In fact, Halloween is a fictional film in Season of the Witch's universe.
Producers John Carpenter and Debra Hill wanted to turn the Halloween brand into an anthology of unrelated installments that all took place on or around the holiday. Apparently, they were hesitant to work on an another one in the series, and this was the only way they'd do it. This may have worked better had they not have already made Halloween II, thus firmly associating the name with the character of Myers, as well as actors like Jamie Lee Curtis and Donald Pleasance.
Fans were disappointed that there was no Michael, and reviews were negative. It took six years for the franchise to come back with Halloween 4: The Return of Michael Myers. They made sure to put him in the title to make sure audiences knew they wouldn't be disappointed again.
I wasn't alive at the time, so I knew before I watched it that it was an oddball in the series and that Myers would come back for more movies. If you just think of it as its own thing, it's not that bad, but certainly not that great either.
Not only does Season of the Witch have a new continuity, it also represents a genre shift for the franchise as while it is a horror, it's also science fiction.
Stars Tom Atkins and Stacey Nelkin give passable performances, but I can't get over how creepy the age difference between them is, and their relationship feels very forced and sudden. She basically throws herself at him without having shown any previous signs of chemistry or attraction.
One of the positives of the film is that John Carpenter did do the soundtrack, which is always a good thing and sets the atmosphere. Unfortunately, a really annoying jingle also plays throughout that will undoubtedly be stuck in your head for days.