Tammy and I understand the "Circle of Corn" in America and have opted out of it in every way we can, other than enjoying some great fresh sweet corn late in the summer. We didn't know much about this documentary though.
If you've read Omnivores Dilemma your very familiar with industrialized corn. The middle section of Michael Pollen's book is specifically on this topic. King Corn is sort of a movie version of that section of the book. Michael Pollen even shows up in the movie with a number of interview segments.
King Corn follows two guys in there mid-twenties that have roots back to Iowa (great grandfathers) but live in Boston as they relocate to Iowa and grow a single acre of Liberty Corn outside of Greene, Iowa. They go through the whole process of growing it, the economics and the eventual products made from it. It's a good documentary and if you are unfamiliar with the dominance of corn in our food chain it would be a great starter. If you are familiar, it is an entertaining refresher. They have a segment in the movie where they visit the Earl Butz, whose farm policies altered the course of our food chain so drastically.