I've read the book like 10 years ago, but haven't seen the film yet. It's very clear what's going on in the book (well, once you get to the end anyway).
SPOILER IN WHITE TEXT as the normal Spoiler function isn't working for me:
aliens take humans, fatten them up like pigs then eat them. they modify scarlett, who is imagined much less attractive in the books, to help procure the meat
As soon as I saw a sexy-looking Scarlett Johanssen was playing the lead, I knew it was yet another example of style-over-substance.
Culture's eaten itself.