A serious-minded filmmaker, Gillies MacKinnon makes pictures a socially conscious Brit-flick fan probably wants to like more than they can.

He explored the trauma of World War One poets in the worthy but hardgoing "Regeneration", while "Hideous Kinky" saw Kate Winslet stretch the boundaries of permissive parenthood (and audience patience).

His finest, "Small Faces", forgot to be earnest long enough to be entertaining, but "Pure" only manages this intermittently. That elements of it recall François Truffaut's masterful depiction of delinquency and youthful ennui, "The 400 Blows", is impressive. However, it also serves to highlight its deficiencies.

At heart, it is a story about the love between a boy and his mother. His dad dead, ten-year-old Paul (Harry Eden) heads his household, looking after his younger brother and smack-addicted mother (Molly Parker).

She's in thrall to her lover, Lenny (David Wenham), a local dealer and pimp. His "medicine", Paul comes to realise, isn't doing any good. When a friend dies through an overdose, he sets about trying to get his mum clean, and pals up with worldly waitress Louise (Keira Knightley).

A former youth worker and teacher, MacKinnon certainly knows how to handle a young cast. Entrusted with carrying the film, Eden is a belligerent, magnetic presence, while Knightley suggests she's capable of more than sulking prettily.

Of the adults, Wenhan impresses - bringing nuance to a nasty piece of work (and requiring a double take when you realise he was Faramir in "The Lord of the Rings: The Two Towers").

Pity, though, that his increased presence signals a third act lurch away from noodling social realism toward pseudo-thriller territory. There's little of the emotional resonance of "The 400 Blows", as a conventional narrative ties everything neatly, in a too-sweet denouement.

The conclusion undermines a thoughtful treatment of a serious subject, tilting the enterprise towards triteness and sentimentality. Call it heroin cheek.