Twenty-four year old Marnie has bad thoughts all the time and she can't tell anyone. Her mind is XXX-rated and intrusive thoughts are piling up inside her head. She is caught in the grip of an excruciating form of obsessive compulsive disorder - nicknamed 'pure O' - where her obsessions take the form of intrusive sexual thoughts, and the compulsions are unseen mental rituals that deeply affect her daily life. Something. Has. Got. To. Give. At breaking point, she packs a bag and, with no plan, jumps on a coach to London. In the capital, Marnie soon discovers she's not the only one who's lost.