Documentarians Steve Brown and Jessie Deeter make an enthusiastic case for the annual Labor Day weekend gathering of the techno-hippie tribes in Nevada’s Black Rock Desert, to which they have complete access. Once in awhile the doc sneaks up on something controversial about the event, its organizers, and its followers -- but mostly it deals anecdotally with what has become a carefully packaged spectacle, a curious combination of visionary creativity and frat-party hedonism. The best segments profile ordinary BM craftspersons, the ones who build the gigantic structures, then blow ‘em up. And then clean up behind the crowd (90 min.).