“Diary of a Teenage Girl” opens Friday at Point Cinemas, Star Cinemas and Sundance Cinemas. R, 1:42, three and a half stars out of four.

Don’t be too fooled by the Sundance-y quirky look of the poster for Marielle Heller’s terrific film, with the protagonists sitting in front of wallpaper that Wes Anderson might have picked out for them. Although Heller’s debut does have some stylistic flourishes, this is not an exercise in style, but a refreshingly honest film, both funny and sad, about growing up female in all its messy complexity and wonder.

That frankness begins in the first scene, where we see babyfaced 15-year-old Minnie (Bel Powley), wearing a backpack and a Mickey Mouse T-shirt (the R. Crumb version) happily walking through a park in 1976 San Francisco. Why is she so happy? She’s just had sex for the first time, with her mother’s 35-year-old boyfriend Monroe (Alexander Skarsgard).

This is, of course, terrible. But Minnie doesn’t know it yet — she thinks she’s found a shortcut to womanhood. And the power of Heller’s film (adapted from a graphic novel by Phoebe Gloeckner) is that it approaches everything from Minnie’s perspective, allowing her to the dignity of  her delusions and her contradictions, allowing her to stumble and make mistakes and survive them. In the end, she’s a stronger, more mature person, not because of anything that happened to her or what an adult said to her, but because of how she worked through it on her own.

Her dalliances with Monroe stumble forward in an on-again, off-again way, as Minnie tries to figure out how she feels about them. Her mother (Kristen Wiig) is oblivious, a glamorous day-drinker who watches the Patty Hearst trial on TV and shouts “I know how you feel, Patty!” at the screen. Minnie is pretty much left on her own, to hit parties and bars with her friends, to try different boys on for size, to experiment and to explore.

None of this comes without cost, but “Diary” refuses to either condemn or celebrate Minnie’s behavior, instead preferring to try and let her explain what’s going on in her head. To this end, the film is aided greatly by Powley’s appealing direct performance, using her girlish exterior (she’s 22 in real life) to her advantage as she portrays Minnie’s unsettling lurches into adult experience. Heller illuminates her tumultuous inner life with the aid of illustrations, reminiscent of ’70s underground comics, that explode off the screen, as Minnie’s drawings on her bedroom wall come to life, or her desire causes feathers and flowers to erupt on her body.

Minnie’s journey is a powerful one, and an ultimately empowering one, although certainly one I don’t think most teenagers are ready for. Better to take Minnie’s own advice at the end of the movie, when she says of her story, “This is for all the girls, when they have grown.”