In 1970s San Francisco, a 15-year-old artist enters into an affair with her mother's boyfriend. (  / Sony Pictures Classics)

This summer has presented a bracing tableau of women challenging assumptions about female libido, identity and double standards, from Amy Schumer’s (mostly) unapologetically promiscuous party girl in “Trainwreck” to the mouthy transgender heroines of “Tangerine” to Meryl Streep’s hard-rocking absentee mom in “Ricki and the Flash.” With “The Diary of a Teenage Girl,” filmmaker Marielle Heller and actress Bel Powley deliver the most artful and politically potent addition to the mix, a sexual coming-of-age story frank enough to be discomfiting and disarming in equal measure.

Powley makes an impressive debut as Minnie, a 15-year-old girl living with her younger sister and mom in 1970s San Francisco. As “The Diary of a Teenage Girl” opens, Minnie is striding confidently across a city park, declaring in a voice-over that she’s just had sex for the first time. Within a few moments, the bravado and unfamiliar flush of desirability have given way to creeping self-doubt, quickly squelched by the incipient flutterings of her own nascent power. Fear, exhilaration and ambivalence have rarely registered so palpably and in such rapid succession.

The story of how Minnie happened to have her first sexual experience on that day, and the lengths she winds up going to in order to test the boundaries of her own hunger, ambition and curiosity, is no doubt the stuff of most parents’ nightmares. But one needn’t have first-person experience with, or even approve of, the extremes Minnie pursues to appreciate the honest, forthright way Heller and Powley present a journey that, stripped to its most basic emotional elements, is timeless and universal.

Filmed in rich, coppered tones refracted through the haze of dust motes and wisps of pot smoke, “The Diary of a Teenage Girl” is steeped in the drugs, music and freewheeling, pre-HIV hedonism of its time. Minnie’s mother, Charlotte (Kristen Wiig), runs a household of fuzzy boundaries, paying little attention to Minnie and her sister, Gretel (Abby Wait), partying nights away with her friends and, at one point, suggesting to her boyfriend, Monroe (Alexander Skarsgard), that he take Minnie to his neighborhood bar instead of her. It comes as no surprise that Minnie and Monroe embark on a relationship, which Heller depicts as understandable, if woefully ill-advised. To the filmmaker’s great credit, “The Diary of a Teenage Girl” goes where “Lolita,” “Smooth Talk,” “Thirteen” and “An Education” have gone before — just to name a few — and claims it as new territory. Here, the adolescent protagonist isn’t the projection of lecherous fantasies, moral-panic anxieties or wistful nostalgia, but a full-blooded human being, her sense of agency fully operational, even when it’s powered more by confusion than a clear sense of purpose.

Bel Powley and Alexander Skarsgard in “Diary of a Teenage Girl.” (Sam Emerson/Sony Pictures Classics)

Is Minnie in love with Monroe? Does having sex mean she’s finally an adult? At what point do the feminist tenets of sexual liberation begin to take on the sobering, slightly icky contours of exploitation and self-loathing? Heller deftly threads the audience and her heroine through these questions and many more, as Minnie not only begins to define herself as a physical being, but also as an artist. “The Diary of a Teenage Girl” is based on an illustrated novel by Phoebe Gloeckner, and Heller suffuses her movie with beautifully rendered drawings and animations illustrating Minnie’s often distorted internal monologue. Tellingly, when she’s feeling most empowered by her newfound avidity and unruly appetites, she imagines herself as an Amazon, bestriding San Francisco like a monster of her own most audacious yet still tentative making.

With her Bettie Page bangs and watchful stare, Powley makes a transfixing Minnie, who ricochets from moon-faced naivete to wry knowingness at the blink of a sea-green eye. Wiig is just as assured as the abstracted, somewhat ditzy Charlotte, who can’t perceive alarm bells even when they’re clanging right under her frequently coked-up nose. Skarsgard’s Monroe, a 35-year-old man-child who under different generic circumstances might be right at home in a Judd Apatow comedy, isn’t so much the villain of this piece as one more misguided seeker whom Heller treats with more amused compassion than disdain. When Minnie and her best friend, Kimmie (Madeleine Waters), begin flirting with their own increasingly perilous dark sides, his interest in Minnie begins to feel almost reassuring in comparison.

“The Diary of a Teenage Girl” does get dark, and it becomes painful to watch Minnie continually try to replace the gaze and touch of a distracted, sneakily competitive mother with the embrace of those who are clearly unworthy of her. Heller adroitly heightens that central psychological drama, even when it’s kept largely off screen while Minnie plunges heedlessly into her next misad­ven­ture.

Whatever that escapade is, it will end up as fodder for her tape-recorded journal, or maybe her confessional drawings, initially perfumed with hearts-and-bluebird notions of love and happiness, and ultimately inspired by her artistic heroes R. Crumb and Aline Kominsky. One of the finest sequences of “The Diary of a Teenage Girl” features a cartoon version of Kominsky giving advice to Minnie as she navigates the intoxicating streets of San Francisco. While she’s trying to find her voice as an artist or control her body as a woman, she’s also trying to ascertain where that crucial, invisible membrane is between her and the rest of the world — whether it should be tough or vulnerable, thick or thin. By the film’s triumphant final scene, it’s clear she’ll be able to figure it out. To paraphrase Minnie after a particularly pivotal chapter of her sentimental education: It’s still just skin.

R At area theaters. Contains strong sexual content including dialogue, graphic nudity, drug use, language and drinking — all involving teens. 102 minutes.