Ava DuVernay was finishing up her civil rights drama, Selma, when she got a call from a Netflix executive: Would DuVernay be interested in making a documentary? DuVernay, who had previously directed two docs, didn’t need to mull it over: “I knew I wanted to do something about prisons.”

Having been raised in Compton, Calif., DuVernay said she’d long been interested in race and criminal justice issues, and had even studied African American history in college. “I was always disturbed, fascinated, and furious [about]…the prison industrial complex,” she said at a press event.

On Friday, Oct. 7, DuVernay’s new, incendiary documentary about race and mass incarceration, 13th, debuts on Netflix nflx . (It also will have a limited theatrical release.) Named after the 13th constitutional amendment, which abolished slavery except as “punishment for crime,” the doc uses archival footage and expert commentary to make the case that slavery hasn’t disappeared from the U.S.—it’s evolved into our modern system of mass incarceration, one in which many prisons are run by for-profit companies and prisoners can be paid a pittance to work for corporations.

Coming at a time when Black Lives Matter and police bias are being hotly debated, 13th was the first non-fiction film to open the prestigious NY Film Festival in its 54 years. The movie received a standing ovation at a press screening attended by this reporter, and it’s already being touted as a possible Oscar nominee. “We didn’t choose a documentary,” NYFF’s director Kent Jones said at a screening. “We chose a film that happened to be a documentary.”

The 13th constitutional amendment was ratified in 1865 and stated: “Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.”

Interviewing noted legal thinkers like Michelle Alexander (author of The New Jim Crow), the documentary argues that this “loophole”— allowing forced labor for criminals—enabled resentful white society to imprison black citizens on minor charges and put them to work.

Since then, the film argues, a variety of measures—from Jim Crow laws to President Richard Nixon’s “war on drugs” and President Bill Clinton’s “three-strikes-you’re-out” legislation—have served to send increasingly large numbers of black men in prison, and several legal scholars and activists interviewed on camera suggest a profit motive at work, as well as racism. Corporations have reaped profits off the privatization of prisons and prison labor; some prisoners have gotten paid as little as 12 cents an hour, doing work for corporations, like Victoria’s Secret and Walmart.

The film charts the explosive growth in America’s prison population; in 1970, there were about 200,000 prisoners; today, the prison population is more than 2 million. Although the U.S. has just 5% of the world’s population, it has about 25% of the world’s prisoners, and about one in three prisoners are black men. More than 60% of the people in U.S prisons are people of color.

“I cried a lot making this movie,” DuVernay said at a meeting with the press last week. In addition to interviewing leading legal scholars and activists, like Angela Davis, DuVernay said she reviewed about 1,000 hours of archival footage, including of images of lynchings, cellphone videos of police abuse, and The Birth of a Nation, the 1915 D.W. Griffith film that glorified the Ku Klux Klan (and was screened at the White House for President Woodrow Wilson).

Ava Duvernay (Director)
Ava DuVernay at the NY Film Festival. Marion Curtis/Netflix

Increasingly, many Republicans, as well as Democrats, are pushing for criminal justice reform and reducing the prison population, particularly of non-violent offenders. But DuVernay’s film points to a profit motive even in that, noting that there are moves to use GPS and other electronic-tracking devices on parolees which could turn poor, black communities into “open air prisons.”

The film also takes aim at both Hillary Clinton and Donald Trump; the movie criticizes Bill and Hillary Clinton for supporting the 1990s crime bill that led to a jump in the prison population. But one of the most chilling scenes in the movie comes when Trump rallies—with angry whites getting violent with black protesters—are juxtaposed against archival clips of civil rights protesters. “In the old days,” Donald Trump is heard saying, approvingly, protesters would be “carried out on stretchers.”

13th, however, doesn’t offer any policy solutions. The goal was to show the larger context, DuVernay said, “so that we’re not living in this fog of ignorance anymore.”