Imagine: one story occurs on the city streets of any urban metropolis and another inside a submarine at the bottom of the Mariana Trench. Clearly, the smaller world is in the sub. And because the sub is so specific, we have the opportunity to really experience it in all its complexities.
Take James Cameron's Avatar as an example. The story takes place in 2154 on Pandora, the lush, Earth-like moon in the Alpha Centauri star system, inhabited by the Na'vi, a ten-foot-tall blue-skinned species of sapient humanoids, who live in harmony with a landscape filled with bioluminescent flora, six-legged predators, and floating mountains. That's new. That's different. That's gold.
Never mind the fact that the story is one we've heard before. Avatar is Dances With Wolves in space, Dances With Wolves is Pocahontas on the Western Frontier, and Pocahontas... well, history books say it's a true story.
It doesn't take much to discern that the story in all three films is exactly the same: a soldier from the civilized world is thrust among a native culture, falls in love with the chief's daughter, bonds with the tribe's people, but is forced to rejoin his old world or fight against it with his new native family.
But as screenwriters, we're often telling the same story again and again. Imagine: Romeo and Juliet on Mars; Romeo and Juliet in the Sahara; Romeo and Juliet as Build-A-Bears at the County Fair.