Pure review: Rarely has a filthy mind seemed so squeaky clean

Marnie, the young protagonist of Pure (Channel 4, Wednesday, 10pm) must be sick in the head. Why else would her mind flicker with such luridly graphic thoughts and cascades of inappropriate imagery?

Like imagining her boss naked, for instance, or milking her mother’s breast, or seeing the pleasant get-together of her parents’ anniversary party suddenly erupt into a bacchanalian orgy?

For a young college graduate living in the Scottish Borders these intrusions have been a source of ceaseless torment since the age of 14; so gratuitous and shamefully prurient in their carnal imagery that they might as well be programmed as a late night Channel 4 show.

“Sorry if that made you a bit sick in your mouth,” goes Marnie’s voice over, following the debacle of the wedding anniversary, the last straw before she abruptly flees to London in search of answers. Her imaginings, she insists, are relentlessly sexual: “Sex that gets you arrested, sex.”

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Yet here the show must be coy. The book on which Kirstie Swain's adaptation is based, by Rose Cartwright, can attest to much worse, describing an experience of "Pure O" – a variant of Obsessive Compulsive Disorder – that made her worry about fantasies of bestiality and paedophilia. The screen, an obviously more graphic medium, may judder with images in the service of black comedy, but some thoughts cannot be depicted.

That makes it difficult for the TV show to relate just how severe or disturbing Marnie’s disorder really is. “They’re not sexy, they’re disgusting,” she says of her thoughts to a fascinated recovering sex addict in a later episode. Here, though, they come across as something closer to an adolescent cocktail of confusion and concupiscence.

Maybe that’s the show its makers would rather offer, though: a millennial coming-of-age story. In Charly Clives’s charming performance, Marnie is a rather adorable, big-eyed naïf in the big city, prattling and fumbling through a clumsy hook-up at a lesbian bar, unsure if she is gay; drunkenly going home with a hip media type, Amber (Niamh Algar); feeling instantly more attracted to Amber’s super-chill, lovelorn flatmate Joe (Anthony Welsh).

As with The Bisexual, another comedy of self-discovery in the city, Pure concentrates on street-level London and, whisking Marnie into a magazine internship, its media meniscus.

"Do I want that?" becomes Marnie's repeated question for herself, never quite sure if her lurid daydreams translate into genuine desires. That may be the right question for a sex-saturated culture too, so accustomed to titillation that it is unlikely to find Pure's assurances of depravity and flashes of full-frontal nudity in the least bit shocking. (Pure O, a clinically contested term, is now arguably the disorder of the world.)

The surprise, then, may be how fundamentally innocent Marnie’s journey really is, looking for a place she belongs. Rarely has a filthy mind seemed so squeaky clean.