Margaret Brown’s “The Order of Myths” is a haunting and important documentary about modern-day American segregation. The racial divisions she captures have been allowed — and encouraged — to flourish under the guise of tradition. Opening today at IFC Center, it is the kind of illuminating work that sends audiences stumbling home in a wide-eyed state of astonishment. The film is a story about a citywide Mardi Gras celebration, but this is a Mardi Gras unlike any other in the nation.

Mardi Gras is associated with New Orleans, but the tradition started in Mobile, Ala. The first parade marched through the streets of Mobile long before New Orleans took up the holiday. During those years, not so unexpectedly, Mardi Gras was composed of two parades: one organized by the local white population, and one organized by the local black population. Arranging different parades, electing different kings and queens, and throwing different galas as a way of celebrating, the dual communities represent a spectacle of segregation in contemporary America.

Ms. Brown, whose own grandfather took part in the white Mardi Gras years ago, titles her movie “The Order of Myths” after the symbolic finale of the parades. Near the end of the line, as the secret “mystic” society brings up the rear, there is a battle between Death and the Devil atop one float. It is a symbolic clash: The Devil is defeated, so the party can commence.

During this documentary, Ms. Brown is frequently informed that there’s nothing wrong with the established order. Everywhere she turns, someone says that this is the way things have always been, and the way they will always be.

And sure enough, as Ms. Brown travels back and forth between the black and white communities, she witnesses the same elaborate, intricate, and even silly preparations made by both groups. There is the election of a king and queen, which brings honor to the respective families. There are secret organizations that control the planning and organize the separate Mardi Gras pageants.

The black and white parades are scheduled on the same day, but at different times of day. The same floats are used in both parades, but they are decorated differently; the whites use the floats, then transfer them to the blacks, who pay for the usage and renovate the designs before marching. In every regard, the same big production is kicking into gear all over town, by two groups of people who will never cross paths during the most festive time of year.

Recording the festivities with an exhausting thoroughness, Ms. Brown is there at the key moments, to capture the small, but crucial snippets of footage that prove just how deeply segregation has taken root. She talks to an elderly white gentleman who has no patience for all this talk of integration. The white Mardi Gras queen describes her discomfort in dividing the festivities and breaks down the reality of the insulated white universe: The whites in her community show a willingness to work with black businessmen, but stop short of embracing them as friends. No matter how much she wants to change things, she seems to realize this is going to be a long battle. And donning her crown, she goes on with the whites-only celebration.

Then there’s the black mayor, who is more than willing to attend a special whites-only ceremony, playfully handing over the key to the city to the white king for a day. Even when it comes to the parade’s floats, we see the economic system that has become a way of life, with the whites buying the floats and then leasing them to blacks.

In major and minor ways, “The Order of Myths” shows how rigid these divisions have become. Still, Ms. Brown allows us to witness the first act of integration that stuns this polarized community. When the black king and queen of Mardi Gras make an appearance at the all-white Mardi Gras ball — invited by the white organizers, who have supposedly made similar, unaccepted gestures in years past — one can sense the hushed drama as these two worlds collide in a rare, public way.

As the black members of the royal court are embraced by an ovation from those attending the white ceremony, and as the white king and queen return the favor, taking part in the black celebration, “The Order of Myths” takes an anticlimax and turns it into a triumphant finale. There is little drama to be found at either gala. There are no boos or jeers. And while it’s clear that this two-tiered Mardi Gras world will not change overnight, Ms. Brown recognizes these gestures for the profound baby steps they are. Cutting back to one last shot of the Mardi Gras parade, Ms. Brown returns to the movie’s central theme: the “order of myths,” marching down the street, the cackling Devil dueling with Death. They fight as the crowd cheers, and then Ms. Brown cuts to the final shot: The float is far down the road, and the Devil is nowhere to be seen. The brass band is bringing up the rear, and it is time to celebrate.

ssnyder@nysun.com