I’ve written two essays on plot, both of which can be found on this website. In the first installment, I gave my definition of the term “premise”, and it goes a little something like this: “…it is the springboard or gist upon which a plot can be built. It’s the true nub of story. Here’s a premise most of you will probably recognize: "A killer shark stakes a claim to the waters off a New England town, and the local sheriff is forced to deal with the problem.”
This example (taken, of course, from the movie Jaws) is brimming over with conflict. In fact, it’s a tight summation that implies both a lead character and a problem to be solved. Premise then, at least as I define it, is an equation: Character + Problem = Plot (otherwise known as “Story”). Crafting a viable premise early on and stating it cleanly can carry the writer a long way toward a viable piece.
This isn’t an original idea, this notion of the Premise Statement. Most books on screenwriting espouse some version of the “logline”. A logline is the TV Guide version of a movie’s plot — something along the lines of “A young farm boy must save the galaxy from an evil galactic empire and its doomsday weapon”. The concept of a tight summation as a tool for the writer was probably hatched by Syd Field, the granddaddy of screenwriting gurus. Call it what you will — either premise or logline — this capsule description is a great first step in story construction, and it is built, again, around the equation Character + Problem = Plot.