But just because everybody knows what the game is about doesn't mean you can't or shouldn't pull huge surprises within that premise.

Having a fantasy game where the heroes are seeking an evil necromancer turn into a post-apocalyptic setting where the necromancer is a nuclear scientist is one thing. Having the necromancer turn out to to be the long-trusted mentor of the party's wizard is another.

Transcript

GM: Anakin, you arrive at the correct position, except... no Naboo.
Anakin: What do you mean? Where is it?
R2-D2: Scanning...
[SFX]: < boppy bop >
GM: It's now in orbit around a nearby gas giant. The tides are causing increased volcanic activity.
Padmé: Cool!
Anakin: What? How?
GM: The Trade Federation conquered Naboo with little resistance. They moved it. Now they're basically smelting the entire planet.
R2-D2: To build the Peace Moon!
Anakin: I have a very bad feeling about this.
R2-D2: Wait, isn't Naboo basically a giant sponge full of water?
GM: It's not that full of water; that'd be stupid. It's still 99% rock.
Yoda: Did the Gungans evacuate?
GM: R2's scanner shows pockets of survivors. But with no Jedi to lead the defence, many have perished.
Anakin: Oh dear gods. Padmé will be devastated.
Yoda: What about the dinosaur ponies?!
GM: I'm sorry, Sally.
Yoda: Noooooo!!!
R2-D2: On the bright side: Superweapon.