Set against the beautiful but scarred backdrop of Montana, the documentary Dark Money opens like a political thriller.

John Ward, running for re-election to the Montana house of representatives, has been the victim of a vicious campaign against him – a series of leaflets attacking his credibility issued by a shadowy organization, seemingly independent of his opponent.

The mysterious attack propels the film into a deep dive into campaign finance in America – which was changed forever by the supreme court’s Citizens United decision in 2010.

That decision effectively allowed corporations to plough as much money as they wish into campaigns backing – or equally often attacking – politicians, effectively influencing policies at federal, state and local level.

Montana finds itself at the frontline of the battle against that decision, as its attorney general – and now governor – Steve Bullock fights to prevent the supreme court ruling being implemented in the state. Dark Money’s director, Kimberly Reed, who grew up in Montana, said she felt compelled to make the film.

“When I heard about the Citizens United decision, you could just see that if the supreme court gave a green light to corporations to spend unlimited money in political campaigns, you could just see political power slipping out of the hands of the average citizen and into the hands of a handful of super-rich people,” Reed told the Guardian.

“When it happened in 2010 I didn’t know what to do. I think like a lot of people I was stymied by it until I heard what my home state of Montana was doing in resisting that supreme court decision.”

Montana had a special place in the fight against Citizens United. In 1912, the state passed a law that banned corporations from spending money on political campaigns, giving it some of the strongest campaign finance protections in the country. Montana had experienced a boom in copper mining in the early 20th century, and the associated “copper barons” had been buying up politicians to further their pursuit – decimating parts of the state in the process.

“You just sort of grow up hearing the stories of industrialists coming to Montana. Taking a lot of natural resources out of the [Butte] hill. And then when you drive through Butte today it’s a very different story. The money that was taken out of Butte did not go back to Montana,” Reed said.

In Dark Money, Reed shows exactly how Butte, a town of 30,000 that experienced massive growth in the early 20th century, was affected by those industrialists and their outsized influence. The town’s former Anaconda copper mine is now flooded with water that is “about the acidity level of your stomach acid”, Amanda Curtis, a Democratic state representative for Montana, says. The pit is now threatening to overflow. It’s a result of “literally every level of our political system [having been] dominated by corporate interests”.

Reed interviews an impressive array of Montana figures as she documents how corporate interests are once again dominating American politics.

We don’t just hear from those opposed to corporate spending either – in another example of Dark Money’s similarity to a political thriller, we even have a bad guy: Jim Brown, who is representing the rightwing group American Tradition Partnership. The group sues Montana in an attempt to get the state to comply with the supreme court decision.

Unfortunately for most of the characters in Dark Money, the supreme court rules against Montana, ordering it to comply with the Citizens United decision.

The tracking of the fight at points segues into a commentary on the state of journalism – and the threat to democracy that the increasing closure of news organizations presents. We see the Great Falls Tribune close its Montana bureau and journalist John Adams, who has been investigating corporate money for years, ends up living in his car.

“It wasn’t something that was immediately obvious,” Adams said of the interference in Montana. “But the mountains of evidence continued to pile up and continued to pile up.”

Adams sets up his own news website, the Montana Free Press, and carries on chipping away at the story.

“We’re starting to see an evolution where the public finally understands that there is actual value to this thing that they’ve been consuming online for free for so long,” Adams said.

Despite the depressing subject matter and the seemingly bleak outlook, for Reed, there is hope that the US can change the Citizens United decision.

“I think awareness of this is spreading. I see it taking root, and I see state legislators who are starting to focus on this a bit more,” she said.

“[But] it’s not going to come from the supreme court. I don’t think it’s going to come from the top down. I think it’s going to come from the bottom up.”

Reed hopes that incremental legislation at lower levels can begin that change, as well as public opinion as people become more aware. She draws parallels with the battle for marriage equality in the US, where individual states began to allow same-sex marriage before eventually the supreme court legalized it.

“A city council issues a resolution, and then another city council, or a school board has a problem about spending in elections, then another one follows: state after state after state,” she said.

“One of the things I hope our film does is raise awareness with the public that they can talk about this issue and hold their elected officials accountable. And make sure that they know that people are paying attention and they’re voting on this well.”

• Dark Money is out now in the US with a UK date yet to be announced