Every so often, someone will leak a tape of some well-known musician on an isolated, unsweetened track, with just that person’s pitchy voice or out-of-tune instrument audible. The point is usually that even the most famous pop stars sound awful without a lot of support. But the opposite is true of Merry Clayton, who in Morgan Neville’s documentary 20 Feet From Stardom is heard shrieking, “Rape! Murder! It’s just a shot away!” on a track from the original sessions for The Rolling Stones’ “Gimme Shelter.” Clayton’s vocals on “Gimme Shelter” have always stood out, but heard without the band behind her or Mick Jagger beside her, she’s even more electrifying. Her track lacks for nothing.

20 Feet From Stardom is about Clayton, Darlene Love, Claudia Lennear, Táta Vega, Lisa Fischer, and dozens of other mostly unheralded background singers who sang the hooks and brought the best out of some of the most memorable songs of the rock ’n’ roll era. These singers—mostly female, mostly black—brought depth and soul to the records and tours of George Harrison, Joe Cocker, David Bowie, Sting, Bruce Springsteen, Talking Heads, Lynyrd Skynyrd, and more. A few of them embarked on solo careers as well, usually with limited or no success. 20 Feet From Stardom asks why some of the best vocalists of the last half-century have been known mainly by aficionados. Are there some requirements for pop success that these women can’t muster, or is the system weighted against them?

Morgan Neville is a veteran of the musical bio-doc, having made films about Brian Wilson, Johnny Cash, Stax Records, Iggy Pop, and Ray Charles, among others—some for cable TV or PBS, and some released theatrically. Immediately prior to 20 Feet From Stardom, Neville made the very good documentary Troubadours, about Carole King, James Taylor, and the rise of the 1970s singer-songwriter. As with that film, 20 Feet From Stardom mostly sticks to basics, stylistically: just tastefully shot interviews and archival footage, slickly edited. But Neville does add the occasional artful touch, such as when he follows Fischer around while she runs ordinary errands in ordinary clothes, or when he shoots Love singing in the studio with background vocalists of her own.

Neville’s greatest strength is as a historian, able to sort through a wealth of details to find the pieces he needs to tell one clear, compelling story. In 20 Feet From Stardom, Neville starts with the moment when black singers began to infiltrate a previously segregated industry, liberating background vocalists from dull sheet music and hushed cooing. 20 Feet From Stardom documents the importance of Ray Charles (who showed other artists a new way to interact with background singers) and Phil Spector (who turned background singers into featured performers), and the film goes on to cover how the 1970s and 1980s became a heyday for vocalists-for-hire, with rockers and pop stars alike paying through the nose for an infusion of soul. Neville contrasts that era to today, as cheap digital recording and Auto-Tune have made specialists less necessary.

Neville isn’t really an investigative journalist, though, or a critic. 20 Feet From Stardom raises lots of thorny questions: about whether the industry will only allow so many black artists or women to excel; about whether background singers have been exploited for their sex appeal, or their race; and about whether an overpowering voice alone is enough to merit a permanent spot at center stage. Many of those questions get answered with a shrug, coupled with some vague mention of bad timing and poor packaging. The most persuasive explanatory moment in 20 Feet From Stardom isn’t presented as such: It’s when a trio of background singers sit around a table and belt out “Up Where We Belong” with such force that they effectively kill the song, as they all try to take lead instead of accenting each other. A significant chunk of 20 Feet From Stardom follows up-and-comer Judith Hill as she tries to transition from background to lead, but the tone of the documentary is such that it presents her dream as heroic, not quixotic. Neville strives a little too hard to remain upbeat.

Then again, many of Neville’s subjects are upbeat, because they’ve had time to appreciate that they have a unique and uniquely rewarding job. The ultimate aim of 20 Feet From Stardom isn’t to argue that these women should be stars, but that their contributions as background singers should be better recognized. That’s why the most ingratiating figure in the film is Fischer, who briefly became a solo star with her 1991 album So Intense and its Grammy-winning hit single “How Can I Ease The Pain,” but couldn’t follow that up, and instead took her ethereal voice back into the shadows, behind Sting and the Stones. Fisher isn’t bitter in the least. She seems to share the attitude of Charlotte Crossley, who boasts about all the great songs she’s gotten to sing—“and sing on key.”