The documentary runs for almost exactly seven minutes (coincidence? UNLIKELY!) and contains a first peek at the bad guy you will grow to love and hate in Halo 3. Well, mostly hate, but you will love fighting him.
You'll see what amounts in a way to the first glimpse at Campaign content, but don't get too excited. Everything you see here is a work in progress, and I'm afraid you won't be seeing anything like finished graphics. This is just a chance to talk about the design philosophy and production behind what's going to become a very important part of the next game.