At the LFF: "Shock Waves: Diary of My Mind"

Shock Waves: Diary of My Mind is another artefact plucked from the ever-expanding borderland between TV and cinema. It was shot on digital for a Swiss true crime series by Ursula Meier, whose fine track record (2008's Home, 2012's Sister) and expert marshalling of a superior cast has landed it a surely unforeseen theatrical outing during this year's London Film Festival. This episode places the shock in the series' title upfront: it's the story of a disturbed teenager (played here by Meier's acteur fétiche Kacey Mottet-Klein) who, on a nondescript Friday in 2009, shot both his parents dead after posting a rambling text confessing his crime to a literature teacher (Fanny Ardant) who'd set her class a diary-writing assignment. The bulk of Meier's film unfolds after the boy has been taken into custody and the diary has landed on the desk of a woman already struggling to deal with both guilty thoughts that she may have in some way inspired this act, and the culpability the local authorities - led by short-fused prosecutor Jean-Philippe Écoffey - come to confer upon her.

What follows is a reconstruction in the conventional sense of the term, using lines from the killer's text to describe the run-up to the killings, and to try and comprehend why such a senseless crime took place; yet Meier's radical empathy - a compassion seemingly beyond the paygrade of those who investigated first time round - extends to an attempt to enter the boy's shattered headspace (laying his words over close-ups of uncomprehending faces) and understand the lonely teacher's need to stay close to someone most of us would possibly distance ourselves from after a certain point. It remains pretty exacting viewing - even at 70 minutes, you'd need a fair bit of Toblerone to get through it without ad breaks, and it winds towards an especially painful scene of self-harm - but the approach does get it under your skin, and the closing scenes are unnervingly strong. In its fierce terseness, this Diary cannot help but recall another: Meier's film has the look of a 21st century Bresson fable, and if that director would probably have forsworn Ardant's crumpled glamour (though she's not untouching in her solitude), you sense he'd be at least as fascinated as Meier with Mottet-Klein, roughly hewn yet ever-expressive, credibly dangerous to know.

Shock Waves: Diary of My Mind screens on Fri 12 (ICA, 6.15pm) and Sun 14 (Rich Mix, 1pm).