Margaret Brown’s film achieves the holy grail of documentary: it turns a little local difficulty into a microcosm for the wider world, and renders a slowly-unfolding human drama as tense and absorbing as any theatrical plot. In Mobile, Alabama, Mardi Gras is celebrated in lavish style – two times over. The town’s black community has one parade; the white families stage another. (Black people aren’t actually allowed on the white people’s floats, although they can play in the bands that march behind.) For an outsider, this latter-day remnant of the effects of slavery and segregation is a shocking anomaly. For Brown, however, who grew up there, the archaic traditions of Mobile are familiar and even held in some affection; her investigation of their darker side is therefore warm and personal as well as provocative.

The Order of Myths is a portrait of shared obsession, which emphasises the human need for elaborate social ritual and shared community endeavour; but it also notes the proximity of the past and the enduring rawness of the shared wound of slavery. The city’s 2007 white Mardi Gras Queen, Helen Meaher, is the descendant of a very old Mobile family who are reputed to have owned the last slave ship that docked in Mobile from Africa. The ancestors of black Queen Stefannie Lucas, meanwhile, could have been those very slaves. (States Lucas, without rancour, “My people were on her people’s ship.”)

The precise nature of this historical legacy when the two women formally meet, as part of an encouraging but awkward local effort to integrate the two Mardi Gras courts, creates a tension that speaks volumes about modern America. Brown’s first film as director was 2004’s highly acclaimed Be Here to Love Me: A Film about Townes Van Zandt; this exquisitely-judged follow-up confirms the arrival of an important documentary-making talent.