“Dark Waters,” starring Mark Ruffalo as an attorney trying to punish the DuPont chemical corporation for dumping toxic waste in West Virginia, is a lone-crusader-against-the-corrupt-system film.

Ruffalo stars as Robert Billott, a Cincinnati, Ohio attorney for Taft Stettinius & Hollister, a firm that represents major corporations, including DuPont, one of the world’s most powerful chemical manufacturers. Through personal ties, and against the wishes of his own colleagues, Billott decides to help a lowly cattle farmer from Parkersburg, West Virginia named Wilbur Tennant. Wilbur’s cows have been getting sick, going insane, and dying off at an alarming rate, and he’s convinced it’s because DuPont poisoned the nearby water supply. He’s right, of course, but proving it won’t be easy, nor will establishing a chain of intentionality that might make DuPont liable for cleanup and restitution.

What follows is a detective story with a nice guy lawyer at its center. Robert Billott is convincingly portrayed by Ruffalo as a sort of human version of Droopy the Dog, a cartoon character who defeated flashier, more volatile adversaries by being unflappable, indomitable and polite, and showing up where his foes least expected it. Haynes uses wide shots to emphasize Ruffalo’s modest height compared to looming costars like Tim Robbins (as Billott’s boss Tom Terp). The actor’s turtle-in-a-shell body language further emphasizes that this smart, ethical man is financially, politically, even scientifically outgunned when trying to prove that DuPont has been dumping toxic waste into West Virginia soil, causing cancer, distemper, and rotting teeth in humans and animals alike.

You know going in that you’re going to see a story about how bad things are, thanks to corporate influence over government as well as the economy, but the extent of the corruption is still shocking, highlighting the implicit question: why fight, if the bad guys have already won? The answer, of course, is that you should fight because it’s the right thing to do, and because even the promise of justice is slim, it’s a public service of a more diminished kind to show people how broken the system is.

“Dark Waters” is still a strong and involving, though understated, example of this dying breed of film, resonating with present-day feelings of hopelessness at the brazen corruption on display every day in the United States, and throughout the world. The script is good at showing the hero doing the necessary work to get to a breakthrough, whether by sitting by himself on the floor of a storage room and going through hundreds of boxes of evidence documents, or carefully re-reading a letter from DuPont until he realizes it doesn’t say what everyone else thinks it says. (How often do movies make reading comprehension cinematic? Almost never.)

For all its patience and droll humor, this is an angry movie, rightly so. The most crowd-pleasing moments find Ruffalo transformed into a Jimmy Stewart- or Tom Hanks-level idealistic Everyman, railing against the world’s evils while also taking the time to explain how they became entrenched, and how it’s still possible to fight them, in a small way, at great cost.