It’s possible — though maybe not common — to go through your entire life not realizing that the line Merry Clayton sings in the Rolling Stones’ “Gimme Shelter” begins, “Rape, murder!” 

It’s entirely likely, on the other hand, to go through your entire life not knowing the name Merry Clayton. The young singer got out of bed one night, decades ago, and went in to sing for the Stones while wearing pajamas, with curlers in her hair. It’s impossible to imagine the resulting song without her, especially once you’ve heard her vocals, isolated, ringing through an empty studio.

Clayton is one of the stars of Morgan Neville’s 20 Feet from Stardom, which turns the spotlight on the women (and the occasional man) singing just to the side of the stage. Lou Reed’s “Walk on the Wild Side,” which opens -20 Feet, tells you exactly what the film is going to focus on: “And the colored girls go ‘doo, do-doo, do-doo …’” (One singer points out that that line makes people uncomfortable.) These women, many of whom grew up singing in church, sing the responses to the lead singers’ calls, the harmonies and the hooks that stick in your head. With interviews and performances, old footage and new recordings, anecdotes and memories, Neville builds a case for these singers as stars; theirs are the strongest voices, the most adaptable talents. They come in and sing perfectly, every time, backing the likes of Sting and Bruce Springsteen (both appear in the film, speaking knowledgeably and earnestly about background singers). Their job is not to have an ego, but to make stars shine even brighter.

20 Feet moves quickly through music history, from ’60s girl groups to today’s pop landscape, telling overlapping stories about astonishing voices while quietly insisting that we notice the recurring image on screen: powerful, talented women limited to unsung supporting roles. Some tried for the spotlight and failed. Merry Clayton never found the solo career she hoped for. The gorgeous Claudia Lennear, an “Ikette” who also sang with the Stones, now teaches Spanish. Darlene Love, whose vocals on Phil Spector-produced records were often released under other women’s names, cleaned houses for a time before returning to singing. 

There’s no explaining the alchemy of why one person becomes a star and another doesn’t, but when you listen to the talent on display in 20 Feet, it seems absurd that so few of these women are household names. They know what’s necessary — a mix of talent, ego, ambition, timing and blind luck — but most have learned, sometimes painfully, that there are no guarantees. Some don’t even want fame: Lisa Fischer won a Grammy for her solo work but then never went back to it, preferring the life of a background singer (she’s the one who’s been wailing “It’s just a shot away” in Stones concerts since 1989). Some, like Judith Hill, are still trying, working to balance background singing with songwriting. 

You could make an entire movie about any of these performers, and Neville has made it very clear in interviews that he has tons more material, more singers who didn’t fit this film’s narrative. If 20 Feet from Stardom has a flaw, it’s one that’s impossible to fix: It could be more, bigger, longer; it could be a miniseries, a Behind the Music of the voices you know but don’t necessarily recognize. If you can watch a movie this engrossing, triumphant and bittersweet and not fall down a Wikipedia-Spotify hole of your own research afterward, you are made of sterner stuff than I.

20 FEET FROM STARDOM: Directed by Morgan Neville. Cinematography, Nicola Marsh and Graham Willoughby. Editors, Douglas Blush, Kevin Klauber and Jason Zeldes. With Darlene Love, Merry Clayton, Lisa Fischer, Judith Hill, Claudia Lennear, Bruce Springsteen and Mick Jagger. The Weinstein Company, 2013. PG-13. 91 minutes. Four and a half stars.