Director Cullen Hoback, left, in What Lies Upstream (Hyrax Films)

This deeply informative documentary, which doubles as a fast-paced investigative thriller, kicks off with the news of a corroded tanker at a Charleston, West Virginia chemical plant. The site belongs to Freedom Industries. Initially, filmmaker Cullen Hoback sets out to examine a substance that leaked into the adjacent river, a source of drinking water. This leads to a bigger mystery: Where were the industry regulators hired to prevent such events from happening? The tanker hadn’t been inspected in years, why was that?

Hoback begins by interviewing officials from West Virginia’s Department of Environmental Protection and the local health department, revealing that there is nearly non-existent regulatory oversight. The figure responsible is a toothless DEP, which levies few fines out of fear that companies will take their business to other states. Meanwhile, public outrage continues to simmer as Freedom Industries holds multiple press conferences in which their executives divulge very little. Additionally, the Center for Disease Control also holds multiple contradictory press releases, initially declaring the water safe but later issuing a warning that advises pregnant women against drinking it.

Eventually, Hoback tracks down information about the chemical in the river. However, the true impact of the public health threat remains difficult to gauge, one component being the manufacturer’s decision to keep all data under wraps, citing confidentiality rules as their reasoning. In an increasingly disturbing turn of events, it is discovered that any information about the other compound is completely out-dated. Hoback attributes this lack of transparency to the inappropriately close relationship between private industry and regulatory agencies. As a result, the very system intended to protect the quality of public water, land, and air is fatally undermined by powerful pro-manufacturing forces.

There are times that What Lies Upstream plays more like a work of fiction. Hoback appears in front of the camera, and many of his interview sequences aren’t of the traditional, talking-head, staring-into-the-lens variety, but rather composed of medium shots of the filmmaker with his subject. The film also frequently cuts to Hoback and his reactions to the various discoveries. Overall, he comes across as a likeable, relatable protagonist.

Hoback seems interested in the arcs of certain reappearing interview subjects, such as a high-ranking official of the West Virginia DEP and a physician for the local health department. The lengthy filming period, which took place over the course of two years, allows for the audience to see how these individuals evolve over time, one of the more unexpected, yet compelling aspects of the film. One, in particular, starts off as a corporate tool but gradually awakens to the possibility that Freedom Industries and other manufacturers might not have the public’s best interests in mind.

What Lies Upstream premiered in North America at last year’s Slamdance Film Festival. As seen at Maysles Documentary Center in New York City this week, certain small, but meaningful, editorial changes have been added to the film. Initially, the ending looked ahead with fear at the incoming Trump administration, but it has since been replaced by a recap of everything Scott Pruitt, the current head of the Environmental Protection Agency, has done to undermine regulation on a national level. Perhaps this new denouement is more provocative than the original; the overall content is certainly compelling enough to motivate audiences, or voters, to take action.