Creative writing students and new writers often have real difficulty with identifying the premise of their story. Sometimes that's because they don't have one, or the premise is so weak it won't support much of a story. But often it's because they don't really understand what a premise is.
The premise is similar to the hook, or the one-line pitch, except the point of it is not to sell your story, but to provide its backbone.
|Moon jellyfish, Hans Hillewaert
If your story has no premise, it's like a jellyfish, just flobbling around with no direction or plan in life (apologies to jellyfish who feel undermined by this assessment of their prospects).
|Snake skeletong, dbking
If it has a strong premise and everything sticks firmly to it, with no extraneous curfuffle, it's like a strong, sleek snake.
Most stories are somewhere in between, with a premise but some things (subplots, for example) which branch out from it. But they should branch - they should not float around, only held in the body of the work by the soft tissue.
This is a whale. It has a vestigial leg, the only purpose of which is to prove that evolution is true. It is not connected to the whale's backbone. Make sure your story doesn't have vestigial legs.
Some stories have a premise which doesn't really lend itself to narrative, which is a bit of a shame.
Black Beauty is an example. The premise is 'horses suffer at the hands of humans and there's nothing they can do about it.' So that sets us up with a central character who has no autonomy, but will be buffeted by fate (in the form of capricious humans) and we will just have to see whether he responds by being stalwart and heroic or by being rebellious - and more than likely sent to the knacker's yard as a consequence. It's a moral fable, and a didactic treatise on horse-management. In terms of narrative, it's a series of loosely connected episodes, of the 'and then...and then... and then...' type, some of them with an explicit moral chucked in. (Keep Sundays sacred, don't drink, etc.) This is, as Mary Hoffman pointed out to me, the structure of the misery memoir. I would add: or the saint's life. Hagiography of a horse.
Some stories have a premise with strong narrative potential. The premise of The Hunger Games is that in a future dystopian society, the ruling group keep control by forcing children to kill each other in a reality TV show. The reality-TV-show element has particular resonance for our society because so many of us sit glued to reality TV shows every week, but the central idea of the child sacrifice to keep society in order is as old as the minotaur legend it draws on, and older. (Indeed, I'd say it's a version of what all societies do in trying to maintain the status quo, but that's a digression.)
A story with a strong premise can be fixed. A story with a weak premise, or no premise, is doomed (in today's market - don't write Black Beauty, it's not 1877). Work out your premise before you spend months writing the story - you might be wasting your time.
Do you know what the premise of your story is? Don't say 'it's about a boy who goes on an adventure...' That's a plot summary. What is the question that your book answers? By the way, the question will also tell you whether your book is that elusive thing, 'high concept'. What is the question, and what is the answer? That is your book, in a nutshell: the question is the premise and the answer is the plot. If there is no interesting question, or no plausible or interesting answer, you might like to think again...