At its best when it’s flaunting a grimy, down-in-the-dirt rock ’n’ roll attitude, “The Runaways” is half unhinged, maniacal fun and half treacly biopic convention. Maybe that comes with the territory, but like the titular band it’s based on, “The Runaways” thrives on punk-ass rebellion and unstable supercharged sexuality. The sunny, placid life on the other side of stardom — not so much.

Dakota Fanning stars as Cherie Currie, who wrote the book the film is based on. Fanning’s persona not only doesn’t thwart the film’s attitude, it enhances the tarted-up, jailbait image developed for Currie after she’s picked to front The Runaways, an all-girl rock band.

Started by rhythm guitarist Joan Jett (Kristen Stewart) and drummer Sandy West (Stella Maeve), The Runaways is carefully shaped by wildly eccentric record producer Kim Fowley (Michael Shannon), who sees the band becoming as big as The Beatles.

Early formation scenes seethe with energy and are steeped in trailer trash grubbiness. Fowley doesn’t exactly use slick production methods, and the scenery-chomping Shannon distills his rock-song mentality down to its basest element: “I want an orgasm!” he growls as he builds his masterpiece.

Later, he puts the band through heckler training, raining trash and feces upon his protégés to prepare them for the unfriendly world of rock ’n’ roll. The training comes in handy. Fowley may be deranged, but he knows what he’s doing.

Shannon certainly has the most fun (and easily steals his every scene), but Stewart also makes good as Jett, even if her lower profile in the film is somewhat disappointing. When she can drop the “Twilight” moodiness, Stewart’s able to delve into characters with a natural ease.

Still, the film is stuck on Cherie, and when the band jets to Japan for a series of high-profile gigs, the film constantly looks back on her left-behind sister and sick father in the suburbs. Cherie is oblivious, thanks to the intoxication of stardom and cocaine — the lethal biopic formula.

The film is strong when it lets the music take the forefront, with both Stewart and Fanning doing passable musical work. Writer-director Floria Sigismondi’s music video know-how gives these scenes the stylistic edge, along with the hazy, dingy photography that often bookends them.

That doesn’t stop the film from treading the ground that almost all films of this ilk do, though. Cherie’s inevitable epiphany is both briefly and shallowly realized, and soon The Runaways fades into the background, even though the band endured for several years after her departure.

Despite the film’s title, this is never really a film about the band, with lead guitarist Lita Ford (Scout Taylor-Compton) and amalgamated bassist Robin (Alia Shawkat) barely registering. Currie’s story fits nicely into the rags-to-riches-to-rags motif, but Fanning doesn’t pull off the transitions convincingly enough, and her eventual soft landing back into normalcy hits a dull note.

It may be a nice ending, but it’s not very rock ’n’ roll.