The hard-partying and sexually undiscriminating magazine writer played by Amy Schumer in "Trainwreck" may have raised a few eyebrows, but she looks like Queen Victoria compared to Minnie Goetze, the heroine of "The Diary of a Teenage Girl." Both characters are semi-autiobigraphical creations, and both indulge in their share of sex, drugs and moral transgression. What's discomfiting about Minnie, however, is that she's only 15.

Based on the illustrated novel by Phoebe Gloeckner, "The Diary of a Teenage Girl" is set in the post-hippie San Francisco of 1976, a backdrop that doesn't exactly connote family values. Minnie, an endearing tomboy and budding cartoonist played by a winning Bel Powley, lives in a shambolic apartment with her younger sister, Gretel (Abby Wait), and their divorced mother, Charlotte (Kristen Wiig). The only man around is Charlotte's feckless boyfriend, Monroe (Alexander Skarsgard). After a saucy comment from Monroe stirs something in Minnie, she asks him for sex. He agrees.

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What follows is not the predatory relationship you might expect. Monroe is not a Humbert Humbert -- if anything, he's out of Minnie's league intellectually and emotionally -- but more of a gateway drug. Soon, Minnie graduates to other boys, threesomes, amateur prostitution and a brief fling with an untrustworthy lesbian. By the film's end, Minnie has left Monroe in the dust.

Directed by and written by Marielle Heller, "The Diary of a Teenage Girl" is not a cautionary tale. It almost never moralizes. Its bluntly honest tone can be impressive, as when Minnie, after a particularly unsafe exploit, says simply, "I don't think we should have done that." At other times, though, the film's wry voice-over and whimsical animations (Minnie's comics often come to life) trivialize the material. There's also a nagging sense that we're not getting the whole truth about what would drive such a young girl to advance so quickly and so destructively.

Still, there's something bold and almost confrontational about the way "The Diary of a Teenage Girl" presents the fraught subject of female sexuality. Whatever judgments you pass on Minnie will be your own.