When Lady Gaga first ascended to supernova celebrity, with the release of “The Fame” in 2008, there was a time when she kept her face hidden, under the elaborate masks she wore in videos and even in interviews, in which she also appeared masked. It was an elaborate act of camouflage, but the ruse worked. She was only 22 years old, but she created an aura of mystery around her, evoking “Lady Gaga” as a larger-than-life character. (When you heard it was actually Stefani Germanotta from New York City, you thought: Seriously, who is that?)

Sia, born in Australia 10 years before Lady Gaga, had a career before hers as well. But when Sia took her solo step in the late 2000s, she didn’t just borrow from Gaga’s mask playbook. She hid her face to the point that mystery became part of who she was. “Sia: Nostalgic for the Present,” which opens tomorrow and is showing in select theaters for a limited period, is an acting film that was originally created for Sia’s 2016 Nostalgic for the Present tour, and throughout the entire film Sia’s face is never seen.

She is right in front of us, standing and singing on a platform to the left of the stage, hidden under a half-blonde, half-black wig with bangs that reach to her lips. There’s an oversized white bow on top of her wig, and she’s wearing a white dress that resembles a nurse’s uniform from the early ’60s. Sia’s best songs, like “Unstoppable,” “Titanium,” and “The Greatest,” are anthems of strength, power, fortitude, and invincibility, and watching the film it’s fascinating to see how her facelessness fuels that quality. She is an abstract figure, a pair of lips singing, and this creates the feeling that the power she sings about does not emanate from her. She is channeling a force larger than herself: the rising specter of female triumph. She is simply the vessel.

I’m not entirely sure what the two-tone wig means (female duality?), but everyone in the film wears a version of it: the dancers, the actors (Kristen Wiig, Paul Dano, Ben Mendelsohn, Gaby Hoffman), each of whom appears in a dance video, as well as the choreographer, the costume designer, and the offstage crew members. They all wear the wig, which resembles the legendary black-and-white cookie of New York turned into a fashionable emblem of talismanic power. “Sia: Nostalgic for the Present” is basically a series of 75-minute-long dance videos that were designed to be projected on stage during the tour, with live performers mirroring the moves in the videos. The standout star is, of course, Maddie Ziegler, the whirling dervish of a dancing girl who pranced, darted and whirled her way through three of Sia’s celebrated music videos; He was 14 years old when the Nostalgic for the Present tour took place.

This weekend, if you go to “Sia: Nostalgic for the Present,” what you will see is a movie from 10 years ago (and a promotional one at that), so it severely lacks the impact of the new. There’s no getting around the fact that it’s a greatest hits compendium. But even though Sia’s last big hit was a decade ago, she had such an intuitive gift for creating powerfully infectious pop songs that it’s hard to understand why she hit a wall. I think it might have had something to do with the mask/wig/hideout. Pop stars, especially those with extensive careers, are people who have an overriding desire to stand out. While Sia clearly had an overriding ambivalence about it. She was a camera-shy exhibitionist.

On “Nostalgic for the Present,” her voice is commanding and resonates with those soulful inflections that make her sound like Amy Winehouse if Winehouse were being produced by Giorgio Moroder. The dance performance at the heart of the film is the kind of thing that seemed transgressive back then, but now feels like something out of a 2010s nostalgia reel. Maddie Ziegler, who was born in Pittsburgh and got her start on “Dance Moms,” moves her body with a kind of airy, gymnastic aggression, and specializes in facial contortions meant to express the underlying ferocity of Sia’s music (and sometimes it’s just there when the psycho flourishes to mess with the audience). Ziegler is like a mime crossed with Daryl Hannah in “Blade Runner,” but there’s also a touch of Sid Vicious in the way she sneers and curls her lips like a teenage brat who’s off her meds.

The successes remain sublime. I retain a special adoration for “Chandelier,” a song of hedonistic affirmation and underlying ambivalence that, for me, filters through its brilliant use as the centerpiece of the Broadway version of “Moulin Rouge.” I’m writing about Sia as if she’s gone, but as someone who loves her music, I see no reason in the world why she can’t now enjoy a comeback as ferocious as Gaga had with “Abracadabra.” He wouldn’t even need to come out from behind the wig; that would sound like a big hit at the Grammys. But he would have to bring himself to do what he used to do: create a dance hook as bright, fierce and glorious as titanium.