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Theme. Good narrative stories generally have something to tell us about ourselves or our world. This is where the artist gets to really operate, because the theme of a narrative story might not be immediately apparent. It might be under the surface. It might even be the subject of some debate for audiences. But have no doubt that a good story seeks to tell us something about our world, either explicitly or metaphorically through the story. So it might be man's inhumanity to man. And the story may never state this theme, but the filmmaker knows that on some level he has something to say about it. Note that the best expression of a theme often requires the filmmaker to take a stand on it. So "man's inhumanity to man" is not a fully developed theme in and of itself until the filmmaker delivers a film that explores opposing sides of it and finally draws a negative, positive or indifferent conclusion.
Even if you deviate from a single theme in exploring your story, it's worth keeping in mind in order to try to pack the most dramatic punch for your audience. It's certainly one of the more ephemeral aspects of storytelling. Great writers have often declared they didn't know what the theme of their own work really was. But invariably, theme is what readers and viewers end up talking about for years to come: what was that film really trying to say anyhow?