'I had sex today," 15-year-old Minnie Goetze declares, elated, astonished, in the early going of The Diary of a Teenage Girl. Wide-eyed and raging with hormones and insecurity, Minnie - beautifully and bravely played by Bel Powley - talks into a cassette recorder in the privacy of her bedroom, sharing her deepest secrets and yearnings with no one but herself. And her big cat, Domino.

She wonders whether her new boyfriend, Monroe, is thinking about her. And when he's sprawled on the couch in the home she lives in with her mother and little sister, she can't help but wonder whether her mother senses anything. Perhaps Minnie shouldn't really call Monroe her boyfriend, because he is 35 and is spoken for.

He's sleeping with Minnie's mother.

Set in the haze of 1970s San Francisco, when free love and not-quite-free drugs ruled the day, The Diary of a Teenage Girl may sound scandalous, or exploitative, or deeply inappropriate, but the film - written and directed by Marielle Heller, adapted from Phoebe Gloeckner's graphic novel - is none of those things. It is, instead, an honest and personal and unblurred examination (even through that druggy blur) of a tricky voyage into womanhood.

Minnie is as awkward as it gets, marching through Golden Gate Park with her arms a-swinging and her head a-rattling. Sex is on her mind, as is art.

She keeps a notebook filled with drawings. She discovers the comic-book artist Aline Kominsky, whose Twisted Sisters is full of bad girls doing what bad girls do. (Kominsky becomes a literally animated spirit in the movie, tagging along as Minnie walks through the city, her drawings turning into trippy cartoons before her eyes.)

Minnie has a best friend, Kimmie (Madeleine Waters), with whom she hits the town, experimenting as only clueless kids can do. There's a younger sister (Abby Wait) forever poking through Minnie's stuff. And there is Mom Charlotte, played with a perfect air of self-involvement and careless disregard by Kristin Wiig.

As for the sleazeball that Minnie is seduced by, Alexander Skarsgård, as the lackadaisical and lacking-moral-fiber Monroe, brings the character into sharp focus. Who's to say he's not motivated? If it's two-for-one Tuesdays at the bar, he's determined to be there. Skarsgård's Monroe slinks around, splitting his time between Minnie and Minnie's mother. He smiles, he gets stoned, turning on the scruffy charm - almost oblivious to his smarm.

But it is Powley, as the naive and lonely Minnie, with her wobbly self-image and her platform shoes, who anchors the film. A British actress (she's 23 now), Powley finds the aching heart, the bold curiosity, and the giddy libidinousness of this high school girl (heading for academic probation). Minnie comes alive in the most truthful, messy, exhilarating ways.