Which is worse, corruption you can see or corruption you can't? In Dark Money, a documentary about invisible corporate shenanigans in her home state of Montana, director Kimberly Reed makes the incisive case that the latter threatens to sink our democracy outright.

If 98 movie minutes about the subversion of campaign financing isn't quite your idea of beating the summer heat, let me start by saying there's not a dull or dry moment in Reed's briskly paced film about the secret assault on the American electoral and judicial process by corporations whose agenda is nothing less than the dismantling of government itself. Dark Money tells a hair-raisingly specific American tale of illicit power. In other words, the stuff of thrillers since Hollywood time began—except that in this muckraking expose, the graft was real. Mercifully, so was the pushback from a citizenry long-accustomed to corporate chicanery and led by a few savvy activists working very hard for free.

Dark Money opens and closes with a flock of geese flying over a toxic copper quarry. As Reed's story unfolds, we learn that the beautiful birds' mass demise is only a tiny fraction of the wreckage caused by shady money moguls tinkering with electoral campaigns over a hundred years of Montana history. In part because its elected officials are ordinary folks who often double-up as farmers, teachers and the like, Montana is also heavy on a defiant populism that cuts across party lines. Which may be why it's also the only state, according to Reed, to have fought back locally against the U.S. Supreme Court's 2010 rollback of laws preventing corporations and unions using their treasury funds for electioneering.

Skillfully fielding an enormous cast of witnesses and participants, Reed shows how Democrats and Republicans running for political and judicial office found themselves targeted by vicious attack ads from deep-pocketed non-profits with blandly vanilla names like Citizens United or Americans For Prosperity, or strategically crowd-pleasing monikers like Mothers Against Child Predators. No one had ever heard of these shape-shifting advocacy groups, and it was difficult to track the money back to the shadowy ideologues hiding behind them. With the ranks of the working press hamstrung by financial crisis, it fell to a shockingly small band of enterprising freelancers—helped by some highly compromising documents that turned up in a colorfully unlikely location—to uncover a trail of money and influence that led back to wealthy right-wing libertarians. (The Koch brothers come up a lot.)