There is an iconic scene in the documentary film "Bluegrass Country Soul" when the Osborne Brothers whip a crowd of thousands into a frenzy with "Ruby, Are You Mad?," a howl from the doghouse and their signature showstopper. In suits and ties despite the sweltering summer heat, Bobby and Sonny Osborne tear into their instruments with manic fury, as if to prove their status as headliners at this landmark bluegrass festival in North Carolina tobacco country.

There are fiery solos by Bobby on mandolin and Sonny on banjo, and stratospheric three-part harmonies deftly rendered at a lone mic, as “Ruby” plays out in its entirety. It is one of more than two dozen revelatory, full-length performances in the documentary, recently reissued in a lavish box set by its director, Albert Ihde.

Overlooked upon its theatrical debut in 1972, “Bluegrass Country Soul” is a riveting cinema verite portrait of a bluegrass festival when it was still an underground phenomenon with a rabidly loyal fan base of spectator-participants. It documents a uniquely American musical style in its prime and in its element, without pretense or talking heads or axes to grind. For decades, as the bluegrass festival tradition exploded throughout the United States and abroad, the film gained a cult following, especially among musicians.

Bluegrass was a subculture unknown to Ihde in the spring of ’71 when he was scouting locations for a low-budget film about a fictional country singer that he had been hired to direct as his first feature. The project had no screenplay and an unpromising title, “The Duke of Marmalade,” but Ihde, then 27, was a young and hungry graduate of the drama department at Catholic University who’d made several experimental films, as well as a mixed-media stage production of a Tom Stoppard play starring a willowy, doe-eyed Catholic U. graduate named Susan Sarandon.

A chance encounter with storefront posters for a Fourth of July bluegrass festival in Berryville, Va., led Ihde to a bucolic scene of nonstop fiddles and revelry along a bend of the Shenandoah River. Here legends like Earl Scruggs appeared onstage and stayed in the same campgrounds where festivalgoers roughed it and jammed in amateur bands through the night. “It was an eye-opener,” recalls the New Jersey-bred Ihde, who had grown up on rock music and French New Wave cinema. “I was a stranger in a strange land. When I heard the music, I was hooked.”

The gathering was presided over by promoter Carlton Haney, a native Tar Heel with bushy, scimitar sideburns. Haney had launched the festivals in the mid-’60s, using as his models the famous jazz and folk festivals in Newport, R.I., and the star-studded ’50s-era country package shows where he first cut his teeth as a promoter.

Ihde wrote a festival scene with a Haney-inspired promoter into his film script. When funding for “The Duke of Marmalade” fell through, he bought the rights to the screenplay and forged ahead with the project. He took the script to a potential investor who pointed to the only thing he liked about the hackneyed story, the offbeat festival scene, and said, “Why don’t you make a film on a bluegrass festival?”

As it turned out, Haney had scheduled his biggest, most ambitious extravaganza yet, to be held in a few weeks at a sprawling 160-acre site in Camp Springs, N.C. He gave Ihde free rein to film the event. In a quick chain of events, a group of investors appeared, including Red Auerbach, the legendary Boston Celtics coach. Less than two weeks later, Ihde and his crew of 14 had set up shop in an RV nicknamed the Monster at the festival site in a desolate region of red-clay dirt and dying tobacco farms. The crew included cameraman Robert Kaylor, already an acclaimed director of independent cinema verite films including “Derby,” a 1971 documentary on the roller-derby scene, another gritty fringe subculture thriving far from polite society.

Bands like D.C.’s own Country Gentlemen welcomed the attention. One highlight from the film finds the quartet rehearsing before showtime in front of their tour bus, where they’ve attracted a circle of admirers. Kaylor’s handheld camera tracks the Gents in their matching pink and white outfits as they stroll through the fields and around lawn chairs, puffing cigarettes and greeting fans on the way. When they hit the stage, a panoramic shot from behind brings the huge crowd on the hillside into focus. A thunderous roar goes up as the band nails the intricate harmonies on “Matterhorn,” the tune they’d been warming up with: What had just moments before been shared with a few onlookers was now performed for thousands of joyous bluegrass fanatics.

It is one of many scenes that reveal a deep sense of community that sprang up in the early bluegrass festivals, exemplified by the 3½ -day event at Camp Springs attended by at least 10,000 people. “I remember seeing bikers and full-blown hippies sitting next to people like my parents,” recalls Missy Raines of the First Ladies of Bluegrass. She attended the festival as a 9-year-old and appears in the film. “It was a beautiful thing to see people from so many different walks of life enjoying the music.” Remarkably, peace and goodwill prevailed, and for the most part, the festival was a respite from the tumult outside the campground. “The country was split,” Ihde says. “There were people in the streets demonstrating to bring home our troops from Vietnam; Nixon was bombing Cambodia; Manson had just been sent to jail. But at the festival, everybody was getting along.”

When “Bluegrass Country Soul” premiered at the Circle Theatre in D.C. in 1972, it received a glowing review from The Washington Post and had a brief run in limited markets, including drive-ins in West Virginia and Kentucky. Haney helped keep the film alive by making bootleg copies on VHS from a 35mm film print he borrowed from Ihde and peddling them by mail order. In 2006, it was reissued on DVD. Once it sold out, it went out of print, until now.

Today, Ihde, 76, runs a theater production company in Santa Barbara, Calif., after an on-and-off film career that included other documentaries like “The Sun Dagger,” from 1982, about an ancient celestial calendar in New Mexico, narrated by Robert Redford. He says over the years he has had to fend off critics who can’t accept that what many consider the definitive film about bluegrass does not include Bill Monroe, known as the father of bluegrass music. “My answer is always the same: ‘Bill wasn’t there,’ ” says Ihde. “It’s not supposed to be a documentary about bluegrass. We just wanted to show what it was like to go to a bluegrass festival. That’s the film: These incredible musicians were performing for a live audience, and this is how they delivered.”

Eddie Dean is a writer in Maryland.