A book premise serves you throughout the writing process and works for just about any genre (a notable exclusion might be poetry).
In a single sentence you can outline the core of your book. This helps you focus on your central plot in fiction and stick to your main content in nonfiction. The formula is a simple one that works for memoirs, mysteries, self-help, children’s books, screenplays, and more:
[somebody] wants [something] but, in order to get it, they have to overcome [some obstacle(s)].
[Red] wants [to deliver food and medicine to her grandmother’s house on the other side of the village] but in order to do that, she has to overcome [a wild, hungry wolf].
In this sentence, we know who are central character is, what they’re major goal is, and what’s standing between where they are now and where they want to be.
I wanted to become a doctor, but I had to overcome poverty, low academic achievement, and cancer in order to do so. [memoir]
Evelyn wants to solve her husband’s murder, but she’ll have to fight a corrupt judicial system, a two-faced detective, and her own inner demons to get his justice. [mystery / thriller]
Busy people want to write a book, but they have to manage their time, reset how they think about authorship, and bounce back from set backs if they’re ever going to succeed [self-help (The 12-Month Manuscript in specific! 😉)]
Lana wants to go on the school field trip, but she has to figure out how to get her parents’ permission since she forgot her permission slip at home and the bus leaves in ten minutes! [children’s book].
Once you’ve come up with a general idea of what you want to write about, this is the next step in solidifying your concept so you can start your outline / manuscript.
Can my book / screenplay premise involve more than one character, goal, or obstacle?
Absolutely. Some pieces are hyper-complex, to the point that if you start reading more than five pages in you’ll be completely confused. However, keep in mind that the more variables you put into play for the story, the more variables you have to manage. Increasing the number of characters, goals, and obstacles also increases the likelihood that you’ll develop plot holes. In most cases, having one of each is just fine and will be less headache for you as an author.
Can my book premise *gasp* CHANGE?!?!
Of course. It’s yours, after all. As you come up with new ideas, write yourself into a corner, etc. you’re able to make changes to everything from who the main character is to the kinds of obstacles standing between them and their goals. The premise is a living “document” in a sense.
If you keep up with your premise and update it as you make changes, you’ll have a carefully prepared tagline for your book that you can use on the back cover, at the top of your book description, or (if going the commercial publishing route) in your query letter.