Even for freewheeling 1970s San Francisco, Minnie Goetze isn’t your typical 15-year-old. An aspiring cartoonist, Minnie — played by the magnetic Bel Powley in what is rightly being lauded as the breakout performance of the year — roams the city with minimal supervision from her party-girl mother (Kristen Wiig), drinking, doing drugs and failing at school. Oh, and sleeping with her mom’s lollygagger boyfriend, Monroe (Alexander Skarsgård).

That last circumstance, in particular, might not make her Everyteen. But “The Diary of a Teenage Girl” is as rewarding as it is squirm-inducing for its honesty, audacity and artful portrayal of adolescence from a girl’s point of view. Fearless and foolhardy one minute, desperately afraid she’s too ugly to ever be loved the next, Minnie is on a journey that’s spectacular on the surface but universally relatable underneath.

Based on a largely autobiographical 2002 graphic novel by art professor Phoebe Gloeckner, “Diary” was adapted for the stage in 2010 by playwright/actor Marielle Heller to good reviews. Considering her lack of experience writing and directing for the screen, what Heller has accomplished with the movie is nothing short of amazing.

Monroe, somewhat likable despite his icky weakness, is no Humbert Humbert-style creep, just as Minnie is no sex-kitten Lolita. He is, of course, the one in the wrong here as the ostensible grown-up, but hand-wringing over the notion that the affair glorifies adult/minor sex is missing the point.

This isn’t a morality play — like so much of what Hollywood turns out concerning wayward girls — for which the only acceptable outcome is punishment. It’s a coming-of-age experience seen through the eyes of an awkward, unsure teen feeling the heady thrill of an older man’s desire. When he tells her that they should stop, her assumption isn’t that they’re being inappropriate, but that he must think she’s fat.

Minnie goes on to further adventures: dancing on her bed under a poster of Iggy Pop with her BFF Kimmie, watching Patty Hearst on TV and smoking pot with Mom and her pals in the family’s always hopping living room. She writes letters to her idol, underground comics artist Aline Kominsky (wife of R. Crumb, who isn’t mentioned). She uses her superior boudoir know-how to take charge during a hookup with a classmate and, on a disturbing but later regretted whim, services a couple of guys in a bar bathroom with Kimmie for $5.

It’s easy to forget that the story is set more than 40 years ago — until you pause to think about how much more freedom kids had then to make mistakes without their parents or the whole social-media-connected world finding out.

What we can hope has also changed is that audiences are made less uneasy by a girl indulging her libido than they would be if she were a boy.