You wouldn’t guess that a movie about an affair between a 15-year-old girl and her mother’s 35-year-old boyfriend would turn into the feel-good film of the summer, but such is the strange magic of Diary of a Teenage Girl. Marielle Heller’s new adaptation of Phoebe Gloeckner’s 2002 graphic novel finds new life in the coming-of-age genre thanks to a fresh tone, a confident sense of setting, and a remarkable performance by eponymous narrator Bel Powley.

The story is set in San Francisco circa 1976, and much of the film plays as a sort of Manhattan west: an obviously ill-advised relationship between an adolescent girl and a much older man comes across as unsustainable yet not shocking, both because of the girl’s ironclad confidence and because, well, the ’70s. There, though, the similarities end. Woody Allen’s story is told entirely from the man’s perspective, while Diary of a Teenage Girl belongs to its heroine, who is learning to be the protagonist of her own life.

Minnie (Powley) has a loving but tumultuous home life with her mother Charlotte (a superb Kristen Wiig) and younger sister Gretel (Abigail Wait). Charlotte’s co-dependent relationship with her boyfriend Monroe (Alexander Skarsgård) is a roller-coaster of boozy nights and woozy days, and when Monroe starts sleeping with Minnie, everyone’s so busy contemplating the cosmos that they hardly even notice. Though Monroe is emotionally immature, he’s physically experienced, and Minnie’s struggle against the several imperatives to break the relationship off is in part a struggle to find the sexual satisfaction she can’t achieve elsewhere.

Whether you find Diary of a Teenage Girl appalling or empowering, it’s impossible not to be captivated by Powley as she navigates this unsteady terrain with wit and resolve. Writer/director Heller never missteps, and scene after scene connects. The bicentennial setting is evoked not just with precisely rendered production design and pop culture signposts but with a through-running theme of discovery: this is an America where everyone seems to be figuring out just how much freedom they can handle.

The story’s genesis as a semi-autobiographical graphic novel about an artist discovering her own talent is represented with animated interludes that illustrate Minnie’s self-exploration without feeling gimmicky or distracting. (For an example of how to do this poorly, see I Believe in Unicorns, another recent coming-of-age film that attempts a similar approach but ends up being all style and no substance.)

Diary of a Teenage Girl is a deeply humane film, a film that neither valorizes or demonizes its characters—and never condescends to any of them, or to its viewers. Though Powley’s round face, with its deep eyes and upturned nose, goes through a spectrum of emotions, its default expression is hopeful and slightly amused. That’s a good way to go through adolescence, and through life.

– Jay Gabler