Indiana Jones And The Kingdom Of The Crystal Skull (2008)
The Original: The first three Indiana Jones films - starting with Raiders Of The Lost Ark - followed Indy (Harrison Ford) as he attempted to stop evil Nazis (and, briefly, voodoo priests) from destroying the world.
The Radically Different Sequel: Much-belated and much-maligned, the fourth Indy flick picks up in 1957, where Indy takes on Soviet baddies who are attempting to locate the mysterious crystal skulls.
Biggest Change: While the era switch makes sense considering Harrison Ford was considerably older when he made the fourth film, there's also a switch in genre.
Crystal Skull is more of a period sci-fi, while its predecessors were all period action capers inspired by pre-war serials. Also, it was pants.
Poltergeist III (1988)
The Original: A unique blend of Spielbergian family drama and Hooper-an horror as Tobe Hooper unleashes vengeful spirits on the Freeling family.
The Radically Different Sequel: The Freelings are all but forgotten as this sequel transports tiny blonde moppet Carol-Anne (Heather O'Rourke) to stay with her aunt in a spooky high rise.
Biggest Change : Ignoring the obvious (it's, er, crap and not at all scary), the setting's a total shake-up and the family members feel like an after-thought instead of the heart of the film.
S. Darko (2009)
The Original: Donnie Darko , the time-twisty paranoid thriller starring Jake Gyllenhaal as the doomed Donnie, who suffers visions of his own demise.
The Radically Different Sequel: Donnie's sister Samantha (Daveigh Chase) comes to the fore seven years after the first film. She goes on a road trip with a buddy and starts having strangely prophetic visions.
Biggest Change: Donnie's nowhere in sight (well, he's dead) and writer Nathan Atkins does a surface-level reading of the first film in an attempt to create a series of spin-offs.
It fails magnificently - it looks, feels and sounds cheap.
Lawnmower Man 2: Beyond Cyberspace (1994)
The Original: A Stephen King short story is stretched to snapping point by director Brett Leonard. King later sued the filmmakers for using the title, stating it "bore no meaningful resemblance" to his original story…
The Radically Different Sequel: Jobe is resurrected by a crazy megalomaniac who wants him to create a super computer chip (bless) that will link up all of the world's computers. Eeeeeeevil.
Biggest Change: For some reason, though the film's set five years after the first one, the 'present day' of LM2 is in some weird kind of future Earth with flying cars and dystopian cities. WTF?
Weekend At Bernies II (1991)
The Original: Andrew McCarthy and Jonathan Silverman play two goons who mistakenly believe that they're responsible for the death of their boss - Bernie (Terry Kiser) - and pretend he's still alive in order to evade a hitman.
The Radically Different Sequel: A bizarre voodoo-imbued follow-up in which a voodoo curse reanimates Bernie's corpse so that it walks to the place where treasure has been buried.
Biggest Change: The introduction of voodoo to the plot - who the hell smoked up that idea?
The NeverEnding Story III (1994)
The Original: Cult classic directed by Wolfgang Petersen. Bullied young thing Bastian (Barret Oliver) reads a strange book that draws him into its fantastical adventures.
The Radically Different Sequel: A shoddy, cheap-as-hell, made-for-TV threequel that recasts Bastian (Jason James Richter) as the creatures of Fantasia bust their way into our world.
Biggest Change: Rock Biter! What the hell happened to Rock Biter?!
Targeting a kiddie market in a way that the first film never pandered to, this is shockingly watered-down stuff on a shoestring budget.
Unlike the first one, this story really does feel neverending...
Tron: Legacy (2010)
The Original: An ambitious sci-fi that looks pretty dated now but remains weirdly loveable. Flynn (Jeff Bridges) is the computer programmer who gets sucked into a computer where a whole new world awaits…
The Radically Different Sequel: Flynn's son Sam (Garrett Hedlund) attempts to track his missing father down and finds himself in The Grid, competing for the right to be alive…
Biggest Change: Though the basic premise is pretty much the same, the execution is massively different - the CGI in Legacy is gobsmacking and director Joseph Kosinski lends the film a sci-punk feel with the banging electro score and costume design.
The Magnificent Seven Ride! (1972)
The Original: Classic sixties Western based on Japanese flick Seven Samurai. Yul Brynner, Steve McQueen, Charles Bronson and James Coburn star as gunmen hired to protect a village from murderous bandits.
The Radically Different Sequel: Marshal Chris Adams (Lee Van Cleef, replacing Brynner) takes on a gang of Mexican bandits after his friend is killed.
Biggest Change: The original cast has been completely replaced by this point, meaning this is a Magnificent Seven movie by name alone.
The Last Exorcism Part II (2013)
The Original: Disillusioned Reverend Cotton Marcus (Patrick Fabian) is the focus of a documentary that seeks to expose all exorcists as frauds. Then he meets possessed girl Nell (Ashley Bell) and it all goes a bit wrong…
The Radically Different Sequel: Having been dispossessed, Nell attempts to make a new life for herself - but has she really escaped the demon that nearly killed her?
Biggest Change: Ed Gass-Donnelly's daft-titled sequel isn't a found footage film, which sets it apart from its predecessor. (But that's not necessarily a good thing.)
Ginger Snaps Back: The Beginning (2004)
The Original: Canadian horror film following sisters Brigitte (Emily Perkins) and Ginger Fitzgerald (Katharine Isabelle). When Ginger's attacked by a wolf, she starts going through some unusual changes…
The Radically Different Sequel: This third entry in the franchise takes place in the 1800s, where sisters Brigitte and Ginger (the ancestors of the first film's characters) get caught in a werewolf trap once more.
Biggest Change: Like Back To The Future Part III (keep reading), we find ourselves in a completely different time.