Premise statement

This week's assignment in my online writing class is to write a premise statement.  As I understand it, a premise statement is a description of your book in twenty words or less.

I would love to have a classy premise statement.  That would mean a good answer for the question I get most often, i.e., "What's your book about?"

But I am struggling mightily with this assignment.  Part of it is wordsmithing. How exactly do I describe the trapped life my heroine is living?  Horrific? Bad?  Or is the word trapped descriptive enough?

Another part of my difficulty is how to keep the sentence shorter than twenty words.  Up until now, my premise was very short:

A gangster love story set in Prohibition LA.

But those eight words don't give you any notion of main character, secondary characters, plot, inner journey, etc.  So I need more, yet less than everything I might throw in.

Apparently writing a premise statement can help you come up with a title for your book, too.  Which, would be great, because right now it's going by the placeholder title of Great American Novel, Take Two.  Not a sexy title and the cover of the book does not yet scream "Buy me!"

Ironically enough, I have easily written a premise statement for my 2013 nano novel:

First fired, then dumped, a young man takes a road trip across the United States and finds out what's important in life.  (OK, it's more than twenty words, but not much more!)

Nano Update, Day 9:

Plot?  Main character has just been dumped by his girlfriend

Word Count? 16,056