There is hardly such thing as an underground movie anymore — except, perhaps, for the ones that have to go underground because they aren’t allowed to be shown. The rough, grainy, outlaw king of those is “C—sucker Blues.” Not because you can never see it, but because on the rare occasions when you can, it has acquired the aura of an unholy testament: the ultimate down and dirty peek behind the curtain of 1970s rock & roll excess. It’s the time capsule that keeps on giving because it’s still semi-buried.

In 1972, the Rolling Stones recruited photographer Robert Frank to shoot a fly-on-the-wall film of their up-and-coming U.S. tour after the release of “Exile on Main Street.” Frank had created the extraordinary cover art for “Exile,” that tawdry collage of photographs that merged the Stones in all their let-it-loose glory with an homage to the rootsy mysteries of Americana. The band now thought of Frank as part of its renegade team, and so they gave him license to shoot whatever he wanted. (The freewheeling Frank wouldn’t have had it any other way; he even brought along an extra camera or two for any random person in the room to pick up and shoot with.)

It was a brazen act of carte blanche in light of the Stones’ last experience — three years before — with cinéma verité, when the Maysles brothers had caught the dread and horror of Altamont on film in “Gimme Shelter.” That was not a movie in which the Stones came off looking good, and this was the first time they were going to be seen in the States following that legendary disaster. But now it looked like a different world. Altamont was the official stake through the heart of the ’60s, and the question that lingered was: What were the 1970s, rising from those ashes, going to be? Here was one answer: The Stones’ 1972 tour would be built, policed, organized. The band’s relatively new logo of Mick Jagger’s lips was the hippest rock insignia ever designed, but in hindsight it was also one of the seminal early acts of corporate counterculture branding. The Stones were now officially the Greatest Rock & Roll Band in the World, and Jagger strutted and pranced and gyrated onstage like a titan of excess: a madly fluttering satyr-ringleader in eyeshadow and a sequined body suit. The Stones, coming off their greatest album (44 years later, I’d argue that “Exile on Main Street” is still the most staggering rock & roll album ever recorded), were going to ride the bucking bronco of the forces that, in 1969, had gotten out of their control.

But they — and, especially, Mick Jagger — still wanted to be movie stars. There were rumors that Jagger had been up for the role of Alex in “A Clockwork Orange” (in 1968, a petition circulated for him to get the role signed by, among others, all four of the Beatles). But Stanley Kubrick never seriously considered him for the part, and cinéma verité, which coursed through the early ’70s the way that reality TV courses through today, was still Jagger’s instant ticket to big-screen mythology. It was the devil’s temptation he couldn’t resist.

The eradication of the line between public and private space that defines our time began, in many ways, with movies like “C—sucker Blues.” It marked a crashing down of the barriers that even the most powerful celebrities couldn’t control. Once the 1972 tour was over, the Stones got a look at the movie that Frank and his co-director, Danny Seymour (billed in the credits as the film’s “junkie soundman”), were making. It was the movie the Stones had asked for, but it was also more than they bargained for — more squalor and sleaze, more drugs and nudity, more scuzzy backstage reality, more trouble. They may (or may not) have been embarrassed by it, but in 1973 the film created a potential political problem for them. Keith Richards faced drug possession charges, and in “C—sucker Blues” he could be seen nodding out after a concert. There was one explicit sequence in which a groupie shoots up on camera. In “Don’t Blink: Robert Frank,” the enticing new documentary about Frank’s life and art, Jagger is quoted as saying that if the movie ever got released, the Stones would never be allowed back into America. (Given that the year before, they’d abandoned their home country of Britain to avoid the heavy tax burden, they were running out of places to go.) And so, despite having commissioned it, the Stones fought the release of “C—sucker Blues.”

The legal battle stretched on for four years, until 1977, when a court order decreed that the movie could not be shown…except that it could be shown at an “archival” setting, with Robert Frank in attendance. And that could happen only — exactly — four times a year. It’s a legal ruling right out of the stoned ’70s: a little ungapatchka, not really all that logical. But the result is that “C—sucker Blues” is the underground document of rock & roll excess that isn’t entirely lost, and last night I caught it at Film Forum, New York’s legendary independent repertory/revival theater (where it’s playing for one more show tonight). I’d seen it once before, back in the ’80s, so what drew me to see it again wasn’t so much that base-line tabloid voyeurism as an intense curiosity about how the scruffy squalor of the early ’70s would look now. I wasn’t shocked to see that it looks more exotic — more free, more slovenly — than ever. What surprised me is that “C—sucker Blues” now comes off as less a catalogue of sin than a weirdly beautiful artifact, a sculpted slice of reality with a theme that could hardly be more relevant today — that the stars we worship as gods are in the very same boat as the rest of us.

In “C—sucker Blues,” the Stones, along with a motley crew of musicians, technicians, and hangers-on that Jagger, at one point, exasperatedly refers to as those same 39 people he has to travel with every day, do a lot of sitting around, in hotel rooms and backstage and on their private plane. A few of the more sensational bits have been staged, all too obviously, for the camera. There’s a moment when Keith Richards and the Texas-born saxophonist Bobby Keys unshackle a television from its place on a dresser and drop it about eight stories off a hotel-room balcony (it lands with a disappointingly unexplosive thud). This now borderline quaint ritual of rock & roll anarchy — trashing the hotel room, man! — is done with so little spontaneity that you watch it wishing that Keith Moon was there to show them how to do it properly. There’s also a mock orgy on the plane, with a handful of groupies coerced into removing their clothing, that would look even more uncomfortable if it wasn’t a quasi-contrived put-on: Robert Frank, whose voice can be heard off-camera egging on the revelers, trying to orchestrate a little spice for his movie.

What’s more resonant is the moment when Mick Taylor, the band’s handsomely coiffed lead guitarist (he’s like a blonder Jimmy Page), wanders into a hotel room where a guy, a girl, and Bill Wyman are all laying around, naked, on a couple of beds, and Taylor says, “I’ve never seen a hotel room blessed with such limpid ecstasy,” and he then asks for a hit of grass, and the infectiously talky girl mocks the very proper British way he pronounces it (“grahss”). Or the moment when Keith, still looking regally youthful, his decayed front tooth the only hint of the wastedness to come, gets into a daft but complicated conversation with a room-service operator, all because he wants to order some fruit. Or a backstage celebrity crush with Truman Capote and Dick Cavett and Lee Radziwell and Andy Warhol and Ahmet Ertegun, or Jagger and Richards looking for all the world like Hollywood starlets as they get their makeup put on, or the band driving in station wagons down a country road, searching for a place to eat and finding nothing, but still glad they’ve got the breathing room of being off on their own, or Mick hanging out with his wife Bianca in a hotel room, murmuring in sympathy to her about a socialite evening she doesn’t especially want to go to.

The most interesting thing about almost every one of these encounters is how little real talk there is, the communication coming out in stray dribs and drabs. It’s easy to get the feeling that the early ’70s was a time when conversation itself had gone underground. (At one point, Jagger tweaks Dick Cavett for being such a chatterbox.) But Frank, a wildly impressionistic filmmaker, shows a flair for taking the most lackadaisical encounters and crafting them into something pungent and revealing. Often, he grafts sound from one scene onto images from another, and he gives “C—sucker Blues” not so much a shape as a living theme, one that’s very Robert Frankian. The movie is about how life on the road, even when it brims with “hedonistic excess,” is still taking place right here on ordinary planet earth.

“C—sucker Blues” isn’t thought of as a concert film. In lieu of its release, the Stones put out “Ladies and Gentlemen: The Rolling Stones” (1974), a movie of the same tour that does a fine job of capturing them in their post-“Exile” glory. But that film doesn’t have nearly the musical poetry of “C—sucker Blues,” which presents the yin and yang of offstage and onstage as something talismanic, with the Stones as magical beings. There’s one sequence in which the slowed-down section of “Midnight Rambler” (“Well, you heard about the Boss-towwwn…”) features Jagger as a nearly electromagnetic figure, an aureole of red, glorious in his erotic menace, and when the band does a duet on “(I Can’t Get No) Satisfaction” with an almost startlingly cool-looking Stevie Wonder, who opened the tour for them, Wonder takes the song and lifts it higher. It’s an ecstatic sequence. Offstage, the Stones still carry an aura. They seem to exist in their own slightly slowed-down time frame. Yet “C—sucker Blues” presents them, essentially, as trapped in an airless round robin of banality and tedium. It’s only onstage that the Stones, with their Satanic majesty, lift us into a realm of transcendent escape, and themselves too.