In 2010, the Supreme Court’s Citizens United decision changed the landscape of campaign finance in America.

The decision held that political spending is a form of protected speech and let corporations and unions spend unlimited amounts of money in campaigns. But to avoid corruption, the court said the money can’t go directly to candidates; it has to go to independent outside groups. What did that mean in reality? The 2012 documentary Big Sky, Big Money, from FRONTLINE and APM’s Marketplace, offered an on-the-ground look. With that year’s election cycle underway, correspondent Kai Ryssdal traveled to Montana, a battleground over campaign finance at the time, and uncovered startling new evidence of outside interest groups’ influence on local campaigns.