The political documentary "Dark Money" is by turns engaging, exasperating and confusing.

Filmmaker Kimberly Reed ("Prodigal Sons") sets out to shine a light on obscure groups that, with deep-pocketed anonymous donors, have meddled in elections.

The film begins with a focus on the 2008 Republican House primary in Montana. Just days before the vote, a lurid postcard was mailed to voters in one of the districts. "John Wayne Gacy, 'the Killer Clown,' " it read, "sadistically raped and murdered nearly three dozen boys and young men." It also asserted that one of the candidates in that district, John Ward, "believes that monsters like this deserve to live."

Ward, a veteran Republican politician, never had a chance.

The postcard turned out to be bankrolled by dark money, which Republican state Sen. Llew Jones defines for the film as "advertising where you don't know who's paying for the ads." He wonderingly asks: "Who's paying for this? What are they attempting to buy?"

Over 98 packed minutes, Reed answers those questions. Or at least tries to.

It's a tricky, complicated story, one that stretches back at least to 1912, when Montana declared that corporations could not make contributions in state elections. This prohibition was challenged in 2010 when the Supreme Court ruled — in the decision known as Citizens United — that the government could not ban corporate spending in candidate elections, rendering state laws like Montana's potentially unenforceable. Montanans had something to say about that, which is where this movie gets cooking.

Drawing on a raft of experts, Reed explores how dark money became a political tool and why it matters. One of the documentary's strengths is that, as it digs into local politics, it also points to the larger national stakes. As Ann Ravel, a former chairwoman of the Federal Election Commission, puts it: "Campaign finance is like the gateway issue to every other issue that you might care about — whether it be education or tax reform or foreign policy."

Reed has taken on a vital story, which is why it's frustrating that her storytelling isn't better. Some introductory text or explanatory narration would have better helped historically ground viewers, who need to juggle a lot of information.

When Reed introduces some time stamps, the chronology and her approach become clearer. Even so, other decisions keep things needlessly murky. Steve Bullock, for instance, is identified as Montana's attorney general (which he was in 2008), but has been its governor since 2013.

It's rarely a good idea to remake a movie as you're watching it, but it's hard not to think that this one might have been improved by a more straightforward approach, better editing, greater critical distance and some basic background information.