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Well, the answer is a little movie called Halloween III: Season of the Witch. A total piece o' crap, that (incredibly) had nothing at all to do with parts I and II. It put the kibosh on any hopes of Halloween franchise becoming much of a goldmine. This got me thinking of other potentially lucrative franchises that were cut down in their prime by a pathetic sequel.
Here's the Retrospace top 7 sequels that jumped the shark. Each one has 3-4 quick examples of why the movie sucked.
7. Superman III
A. Contradicts 50 years of well-known mythology
B. Richard Pryor, flailing around desperately trying to make this piece o' shit funny
C. A near-complete absence of the major players associated with the earlier films: Richard Donner, Gene Hackman, John Williams, etc. - gone.
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A. Rodney Dangerfield replaced by Jackie Mason
B. Dan Akroyd replaced Bill Murray
C. From an R rating to a PG
5. Jaws 3-D
A. Dialogue like this: ""You tell Shelby Overmann for me he can take a flyin’ leap at a rollin’ doughnut on a gravel driveway, you hear?"
B. Creative desperation: This time it's the original shark's MOTHER! Say it ain't so!
C. The final scene: Look! The friendly dolphins didn't die after all! Praise the Lord! They're alive!!! Hooray!! (loud applause) (children crying)
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A. James Earl Jones outfitted like a witch doctor
B. A giant superimposed locust
C. Richard Burton sweating gallons of perspiration
D. Not a single solitary scare
3. Star Wars Episode 1: The Phantom Menace
A. Lots of little Annie, zero Vader
B. Midichlorians.... what?
C. Jar Jar
D. Jar Jar
2. Halloween III: Season of the Witch
A. Lots of stupid exploding masks, zero Michael Myers
B. The masks are made by a Druid who has rigged them with pieces of rock from Stonehedge strapped with computer chips that are set to go off when a special electronic signal is released.... makes sense right?
C. No horny teenagers, no Loomis, no scares, no Jamie Lee Curtis
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The White Lightning poster above contains everything I want in a movie. Then the sequel came along and took it all away from me.
A little explanation is in order in case you haven't seen the original Gator McClusky movie, White Lightning; easily one of the best hick flicks ever. It's chock full of muscle cars, pretty ladies, moonshine, and senseless sweaty violence. Gator is an ex-con badass from the swamp who answers to nobody. He's a moon shine runnin' son of a bitch who will tag every chick in town guaranteed.
Then came its sequel, Gator, directed by Reynolds, himself (surprise!). No longer a menace to society capable of brutal violence and stealing your women, Gator McClusky is now a sensitive ladies man. Gator went from being a shit kicking hillbilly from the swamp to.... (gulp) John Tesh! Here's a few scenes to illustrate my point.
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B. Gator gushes to his lady, tearfully telling her "I can't look at you all at once. I'm afraid you'll disappear."
C. When he and his cohorts try to steal files from the courthouse, he spends the whole time goofing around like Don Knotts whilst cradeling a pussy cat!
I think you get my point. I'm probably alone in my opinion on this, but I think the Gator movies had great potential, but were ruined by Reynolds' horrible direction.
I could have named Grease 2, but I don't feel there was much of a chance of an ongoing Grease series. And Karate Kid II stunk, but was it really that much different than the original? Same goes for Ghostbusters II and Airplane II - not nearly as good as the first, but not that different either.
Anyway, I'd be interested to hear about some of your opinions on sequels that ruined movie franchises.