A new film based on true events and starring actor Mark Ruffalo sheds light on how one of America’s most powerful corporations contaminated a town’s drinking water with a toxic chemical linked to cancer, reports Time.

In Dark Waters, Ruffalo portrays the real-life corporate lawyer Robert Bilott, who in 1998 became an environmental crusader. That year, while visiting his grandmother in West Virgina, Bilott, who represented large chemical companies at his firm in Cincinnati, was approached by a local farmer Wilbur Tennant. Following the deaths of 190 of his cattle, Tennant was convinced that his land was being contaminated.

The farmer’s land, it turned out, had been contaminated by chemical giant DuPont. As Bilott started to uncover the truth, he became the townspeople’s defender. The film follows his two-decade-long fight for justice and depicts how far DuPont went to protect itself.

Bilott represented the farmer and eventually filed a class action suit on behalf of 70,000 people who lived near DuPont’s chemical plant and whose drinking water had been contaminated with perfluorooctanoic acid (PFOA). For decades, DuPont created the toxic chemical for use in Teflon, the nonstick coating commonly used in pots and pans. (Teflon has been made without PFOA since 2013.)

PFOA is one of a class of chemicals called polyfluoroalkyl substances (PFAS). According to the Centers for Disease Control and Prevention, exposure to PFAS can have serious health effects on humans, including decreased fertility, immune system disruptions and increased risk for certain cancers. According to the American Cancer Society, these include testicular, kidney and thyroid cancers.

In 2017, Bilott won a $671 million settlement on behalf of more than 3,500 plaintiffs who claimed to have contracted diseases such as kidney and testicular cancer from DuPont’s decision to contaminate their drinking water with these chemicals despite allegedly knowing about their toxicity.

But Bilott’s fight isn’t over, reports Time. Last year, he filed a new lawsuit against companies including DuPont-associated manufacturer Chemours. The case is seeking class action status and was first brought on behalf of Kevin Hardwick, a 40-year firefighting veteran who used fire-suppressions foams and firefighting equipment that contained PFAS.

“If we can’t get where we need to go to protect people through our regulatory channels, through our legislative process, then unfortunately what we have left is our legal process,” Bilott said. “If that’s what it takes to get people the information they need and to protect people, we’re willing to do it.”

Dark Waters is currently in theaters across the United States. You can view the trailer in the video above.

For related coverage about PFASs, read “Chemicals Found in Common Products Raise Cancer Concerns.”