Oh those awkward years. They're awkward not just because the titular teenage girl is a teenager, but because she's having an affair with her mother's boyfriend. Most awkward is that she's only fifteen.

In 1976, wide-eyed Minnie (Bel Powley – A Royal Night Out) starts a cassette diary: “I had sex today. Holy shit!” For any teenager, loosing your virginity is a holy-shit kind of moment. When it's with your mother's boyfriend (Alexander Skarsgård – What Maisie Knew), it's on a whole new level. Getting to that moment, and documenting the inevitable fallout, forms the bulk of Minnie's diary entries. What happens around the edges, and the emotional resonance that envelopes them, is what makes Diary Of A Teenage Girl mostly interesting.

I say mostly because this ought to be a balls-out shocking film. She's a teenager, he's a paedophile and any issues of need, desire, commitment, complicity, openness or honesty do not change that simple fact. Yet it is neatly subsumed to become the film's background as this decisive youngster sets out to find what she wants from the world (and have some fun along the way – it's the '70's after all). Things go south on the fast train when Minnie's mother (Kristen Wigg – The Martian) does what every Mom does, and listens to her diary. But any shock value quickly gives way to more fundamental, emotional issues ('Does he love me? Do I love him?') which aren't nearly so arresting.

Aside from Powley's compelling presence, what gives the film staying power is Minnie's burgeoning artistic future – she wants to be a cartoonist, Aline Kominsky-Crumb (partner to Robert) is her heroine. The girl's expressive, frequently sexual, work is convincingly folded into the film's visual story-telling as her characters leap on to the screen with lives of their own: it's cute, witty. Likewise the film's narrative core, Minnie's diary. While steeped in self-consciously 'indie' styling, her voice remains honest and credible (literally and metaphorically) as the diary narrates the film. It's funny, effective.

What Diary Of A Teenage Girl is not is gut-wrenching, nor is it heart-breaking and it really ought to be. In some ways this is the flip-side sequel to The Graduate in which Ben has skipped off, leaving Elaine to bring up their unruly daughter alone. But where that heavily stylised film still pulled us into the heart of the action, this seems content to leave us as uninvolved observers: happy enough but distanced. What we need is Grandma Robinson to shake off their self-awareness and pull us screaming into the centre of Minnie's torrid affair.

// COLIN FRASER

Previewed at the Sony Theatre, Sydney, on 14 September 2015