In the opening scenes of “The Runaways,” Floria Sigismondi’s ode to the all-girl hard rock band, the characters played by Kristen Stewart and Dakota Fanning could be almost any teenage girls living in the Valley circa 1975. Fanning is sitcom-cute in a plaid shirt, miniskirt and knee socks; Stewart sports a T-shirt and jeans.

But then Stewart’s character walks into a rockabilly boutique, dumps a bag full of change on the counter and demands “what he’s wearing,” pointing to a guy in a black leather motorcycle jacket and leather pants. Suddenly, high school girl Joan Larkin has transformed into Joan Jett.

Cut to Fanning as Jett’s soon-to-be-bandmate Cherie Currie, staring into the bathroom mirror with David Bowie’s “Aladdin Sane” album cover nearby. With a partially drawn red lightning bolt on her face, she’s chopping her long blond hair into an approximation of his glam shag.

“It looks really terrible,” says her twin sister, Marie, watching in horror.

“Good,” answers Cherie.

And then the title credits roll.

Sigismondi’s feature film debut, opening on Friday, is more evocative portrait than obsessively factual biopic. (It is based on Currie’s 1989 memoir, “Neon Angel,” a new version of which is being released this week.) But the basic plot of the Runaways story — L.A. teens rise from the glitter and grit of the Sunset Strip to kick open the doors of the rock ‘n’ roll boys club only to discover more than they bargained for — is the stuff that rock mythology is made of. Their tough, sexy, “jail bait” style continues to inspire female rockers from Courtney Love to the Donnas.

They put out five albums — including “The Runaways” and “Queens of Noise” — before breaking up in 1979, and saw as many bass players come and go, but the four primary characters in the film remain who we think of when we think of the Runaways: Jett and Currie, drummer Sandy West and lead guitarist Lita Ford.

Some critics wrote them off as little more than the products of a savvy marketing strategy, but Jett, whose black hair and consistently lean, punk look has earned her fashion icon status, dismisses this idea. “People think that our images were dictated to us by men, and that’s not the case,” she says. “It’s not like [our producer] Kim Fowley sat down and said, ‘Cherie, you’re gonna wear a corset. And Lita, you’re gonna wear shorts onstage.’ We would have laughed! Nobody told us what to wear. People like to think that that’s the case because if teenage girls are being sexual” — her voice drips with sarcasm — “obviously men have something to do with it.”

One only needs to see 15-year-old Cherie Currie in her signature black-and-white corset and fishnets, singing “Cherry Bomb,” to know there was nothing coy about the in-your-face approach. For Carrie Borzillo, author of the 2008 book “Cherry Bomb: The Ultimate Guide to Becoming a Better Flirt, a Tougher Chick, and a Hotter Girlfriend — and to Living Life Like a Rock Star,” ” ‘Cherry Bomb’ is about taking the world by storm and going for what you want.”

It’s hard to believe, then, that these girl groundbreakers — headliners on bills with Cheap Trick, the Ramones and Blondie and revered by later generations of chick rockers — are all but unknown in the age of “American Idol.” Even the film’s lead actresses admit they were unfamiliar with the Runaways when they first read Sigismondi’s script.

So Stewart and Fanning took their cues from the source: Jett, who executive produced the film, and Currie were regulars on the set. Stewart says that without the input of Jett, with whom she remains close friends, “I would have felt like such a fraud.”

Fanning, for her part, did such a spot-on, snarly version of “Cherry Bomb” that the real Cherie broke down and cried when she saw it live.

For Sigismondi, who has made her name with stylized music videos for Marilyn Manson and Christina Aguilera, keeping things raw and real onscreen was a new kind of challenge. “The ’70s are so out there,” she says. “It’s really easy for that to become a caricature.”

The director was intent on letting the rough edges show in her R-rated movie. “Don’t cover pimples,'” she remembers saying to the makeup artists. “Give me bedhead,'” she told the hair stylists.

Carol Beadle, Sigismondi’s longtime costume designer, also steered clear of any kind of modern, glossy polish. “This was a subculture,” says Beadle, flipping through a binder filled with pages from old Tiger Beat and Melody Maker magazines featuring regular Runaways hangouts like Rodney Bingenheimer’s English Disco and other cult Hollywood hot spots. “It wasn’t Beyonce and Rihanna,” she adds. “It was self-made.”

To that end, an original ’70s-era Bowie belt buckle made by Currie was revered as the holy grail of accessories on set. Beadle dressed Fanning in it “as much as I possibly could,” and the actress cites it as one of her favorite pieces.

With 150 costume changes for the actresses, Beadle and her team literally scoured every costume house in town and dug through vintage stores from American Rag to American Vintage, Echo Park’s Lemon Frog to Filthmart to Golyester, turning up tiny leather jackets and studded blazers, football jerseys and zippered jumpsuits; bold polyester print shirts, vests and bell-bottom jeans; sequined tops, scarves and sunglasses; kimonos and Pro Keds sneakers.

Platform boots of every height, color and texture (including an especially fabulous vintage pair of green leather with tan snakeskin trim) were bought, rented and commissioned from Andre #1, a custom cobbler on Sunset who made boots for KISS and the Runaways and still had some of the original lasts from the all-girl band. Beadle had Fanning’s corset made by Trashy Lingerie on Cahuenga “because it seemed the most appropriate place,” she says with a smile.

Viewed as part of an ensemble, the pieces may just read as costume, but many of the individual pieces seem in sync with current trends, including long-collared shirts; high-waisted, wider-legged denim; aviator sunglasses and studs on absolutely everything. A good handful of high-end designers — including Gaultier, Alexander Wang and Dior — even showed corsets on the runway this season.

And, of course, “platforms are huge right now,” says Beadle, who is in the midst of designing her own shoe line. “You’ll see the really big ones we used in ‘The Runaways’ this fall,” she says.

And Fanning is already ahead of the curve. She got to keep a pair of silver platform boots that she wore in the film and adds: “I’m just waiting for the right time to whip those out.”