Marnie is a woman in her 20s whose mind has been ruled by uncontrollable X-rated thoughts for as long as she can remember. Not being able to take it any more, Marnie leaves her small Scottish town and heads to London, where she meets a new group of friends, all with quirks of their own.

This leads her to a diagnosis of a form of OCD nicknamed 'Pure O' - where her obsessions manifest as sexual thoughts, and her compulsions are the mental rituals she performs to quiet them.

With this knowledge, can she find a way to manage the deviant thoughts piling up inside her head?