No one has to see a documentary to understand that large sums of untraceable political campaign contributions are a bad thing. But “Dark Money” does need to be seen because it reveals with fascinating specificity how that crooked system works and details how one state decided to take it on.

That state would be Montana, a key target of dark money forces because it is sparsely populated and resource-rich.

As directed and co-written by Kimberly Reed (with Jay Arthur Sterrenberg), “Dark Money” takes a detailed look at an unexpectedly complex situation that is at the heart of political action today. As Ann Ravel, former chairman of the Federal Election Commission, explains, “campaign financing is the gateway issue of every other issue you might care about.”

Although the film does go off on brief side trips to the precarious status of that commission as well as to a similar situation in Wisconsin, it mostly stays put in Montana, where Reed, a native of the state, followed a range of crises and developments over three election cycles to get the story right.

Montanans care about the role of dark money perhaps more than the residents of other states because of their own fraught history with corporate interests.

Companies like Anaconda at one time played an outsize role in the state’s politics, with results like a flooded open pit copper mine site in Butte that’s still so toxic decades later that geese who land on it die off in droves.

Things got so bad that in 1912 the state passed the Corrupt Practices Act, one of the first in the nation to ban corporate contributions to politics. But the U.S. Supreme Court’s controversial 2010 Citizens United decision killed that and took things back to square one.

Montana is also unusual among states in that it does not have a political class. It has what is referred to as “citizen legislators,” regular people with regular jobs who meet for 90 days every other year.

U.S. Sen. Jon Tester, D-Mont., a working farmer, is very much in that tradition. His appearance at the start of the film dealing with a recalcitrant piece of equipment got delighted applause when “Dark Money” premiered at Sundance (where it won a producing award).

“Dark Money” begins by demonstrating the kind of havoc dark money wreaks on a campaign by funding savage, completely fictitious attacks so close to Election Day that no response is logistically possible.

Typical was the attack on Montana state Rep. John Ward, who was accused, in a mailer supposedly sent out by Mothers Against Child Predators, of being a supporter of multiple murderer John Wayne Gacy, a man he had never met and who had never set foot in the state.

What made this attack so initially inexplicable is that, like many targeted Montana legislators, Ward was no liberal firebrand but a Republican with a reliably conservative record. Who would go after people like this, and why?

“Dark Money” follows the struggle to figure this out on several fronts, most noticeably the tireless investigative journalism of John S. Adams, who changes hairstyles several times in the course of filming but never wavers in his pursuit of the answers.

Also key players were Jonathan Motl, the state’s commissioner of political practices, determined to investigate apparent violations, and Gene Jarussi, a lawyer who came out of retirement to spearhead a case against a powerful politician for campaign finance violations.

As if all this wasn’t complicated enough, “Dark Money” takes further detours like showcasing the battle to get the state Legislature to pass the Montana Disclose Act and the energized campaign that Democratic then-Atty. Gen. Steve Bullock ran on this issue to become the state’s governor.

“Dark Money” is overflowing with examples because the filmmakers couldn’t bear to leave out fascinating stories. Best of all is the reveal, featuring twists and turns worthy of Ellery Queen, of how the people behind the crash-and-burn attack ads were exposed, the way they do their dirty work and what their ultimate goal is.

The result is a genuine political detective story, as involving as it is significant, offering specifics from people who have been there about a modern affliction we ignore at our peril.

“DARK MONEY”

Directed by: Kimberly Reed

Starring: The state of Montana

Rated: Not rated

Should you go? It’s involving as it is significant. ***