If your definition of 'plain' regards the 'open prairie' type of grassland so prevalent in the American landscape, you could write a script encompassing some of the struggles of the early pioneers in overcoming not just the native warlike population, but also disease, the hot summers and freezing winters, the toil of the soil to get crops to grow and flourish there, and the genral labor involved in building new communities from scratch in the wilderness. You could add interest by pencilling in some strong characters and inventing intersting histories and life stories for them. The great plains of America became responsible for feeding the whole country with grain, so your story could have an agricultural theme.

It would be more helpful if you could tell us what thoughts, etc, you already have and we could supplement...

I suppose there are two obvious things to do with this sort of a documentary (I assume you are talking about wars with the Plains Indians in the second half of the 1800s).  You should mention the biggest events -- things like the Sand Creek Massacre and the Battle of the Little Bighorn.  You need to give the basics.

Then I would make up interviews with survivors of these events.  These interviews would give various perspectives on the events.  You could also add the opinions of scholars.