A pun is not a premise; it is a pun. A twist ending is not a premise. A punchline is not a premise unless your story is very, very short.
Premise, for all the things it is not, is difficult to define. It is not like characters, who are the people who walk around in your story, discrete and countable. It is not really about the plot, and it is not necessarily the the world or the magic or the technology. The premise is the thesis of your story–the thing that makes it itself. Characters can be shuffled around, action can be re-plotted, but if the premise changes, you’re writing a different story.
The driving force behind a good story is often a strong, original idea. Short stories are especially bound by this rule because they have so much less space–novels can be an amalgamation, like getting drenched by standing in rain–each idea is tiny, but the overall effect is soaking wet. Short stories, without the leisure of a hundred thousand words, have to be more like a bucket of water upended over the head–all the force, all at once, all concentrated into one smashing idea.
In genre stories in particular, premise is important. I have read some spectacular stories where the genre element is only the setting, or where the premise does not lean on the fantastic, but the majority of short stories in scifi/fantasy take advantage of the fact that in our little corner of writing, there is huge potential for completely awesome premise. There is a lot of sand in our sandbox. Our castles can be splendid and impossible; they are made with opalescent sand, decorated with shells that never sheltered anything on earth, and the gravity is optional. Still, lest you feel complacent, note that this is a knife that cuts both ways. With great possibilities come great expectations.
You simply cannot get away with a hero with a magic sword slaying a dragon.
Beowulf did that in 800CE; the genre has moved on.
Cue the cries of “but my hero and my magic sword and my dragon are different!” Great–that’s exactly where good premise comes from. Now ask yourself “how different?” Are the differences surprising and new? Are they important to your story, or are they just window dressing? Does your premise demonstrate that you, as a writer, are aware of the dragons that have come before you, and show that you have something new to say on the subject?
You must ask these questions, and not just for dragons–elves and spaceships and robots and psychics are all under the gun. If the heart of your story presents itself as naive or uninteresting, every other part of the story will suffer.
Read a lot, ask ridiculous questions, don’t be afraid to declare that something is crap and you could do it better if only you wrote it differently. It is okay to share, steal, and borrow. Ideas don’t spring fully formed from the brain-ether, they coalesce from all sorts of places. Despite what the DRM says, ideas are very difficult to copyright, so grab source material with both hands and use it!
I like to have a pool of potential stories floating around in my head and in my computer’s documents folder all the time, waiting for the opportunity to be given plot, character and words. This winnows out the bad ones, because unremarkable premises become forgotten premises, and worthwhile ideas stick around.
One exercise that I learned from Julie Holderman at Alpha ‘10 was to try boiling your story down to two sentences or less. (There was a no semicolons rule swiftly invoked at the time.) This is a great thing to do for a number of reasons, not only because it helps you to speak intelligently about your writing, but also because it does a very good job of showing you your premise, and demonstrating if it is compelling. Be critical of your summary; is your description specific to your story, or could it be describing anything, including the summer blockbuster? After reading just those two sentences, do you want to be reading the story they describe? It’s much easier to see the answer to these questions about your premise when all of the furnishings of plot and prose have been stripped away.
When we fall down at the ideas level, no amount of writing skill is going to save us. It is a rare critiquer who is brutal enough to tell you that they dislike your premise. Nobody wants to condemn a story as flawed to the core–it’s worse that suggesting a POV switch; bad premise isn’t rewrite territory, it’s Game Over, Insert Coin to Begin New Story. It is a good plan to look at the guidelines for submission to markets if you’re worried that your premise might be weak or overdone. They will not bandy words or be kind when pointing out things they are tired of. Try Strange Horizons’ “stories we’ve seen too often” or Clarkesworld’s submission guidelines.
Not all stories follow the rules–there are exceptions, and they can be amazing. Premise doesn’t have to be the genre element–it can be the emotional starting point of the story, or even the particular narrative structure. Cat Valente’s The Orphan’s Tales are all about stories within stories, and honestly the tales themselves come second to her overarching structure. Kelly Link, almost without fail, writes short stories impossible to fit into a two sentence summary. These authors and others do very well outside of normal guidelines.
Don’t rush into a story with a half-formed idea when you could spend an hour brainstorming a better one. The better your premise, the better you can make your story. It’s important, and it’s worth it. At my first Alpha, an instructor told me, “a novel is about the most important time in someone’s life; a short story is about the most important event in someone’s life.” Put simply, if there’s something more important in the backstory, or half a continent away, or happening to the main character’s sister, write about that.