This is a documentary about gerrymandering, and particularly a citizen's crusade against it in Michigan. Gerrymandering is the process of redistricting your constituents to improve your own (or your party's) odds of winning the next election. This is a bipartisan problem (it's an American film, so I'm using American terms): whoever's not currently in power will claim that only the party in power can do it ... and that's true, but they're perfectly happy to do it when they're back in power.
In this case, we're looking at a nation-wide redistricting effort by the Republicans - which the movie claims (fairly convincingly) was responsible for the Flint water crisis. If you can't be voted out, how can people hold you responsible? Which left the Michigan Republicans making some poor financial decisions, cutting budgets they shouldn't have, etc.
The majority of the movie focusses on the citizen's anti-gerrymandering movement in Michigan, with some coverage of Wisconsin. The movie is helped along by the young and rather charming Katie Fahey who founded "Voters Not Politicians" to get redistricting reform on the ballot. While I have some sense of the urgency of their political cause and totally agree with it, it's perhaps not as urgent to me (a Canadian) as it should be to all Americans. Gerrymandering does happen in Canada, but hasn't reached quite the fever pitch it's at in the U.S. Nevertheless, an interesting movie.