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. It turns out she is actually caught in the grip of an excruciating form of obsessive-compulsive disorder - nicknamed ‘pure O’ - where her obsessions take the form of intrusive thoughts, often sexual.
Witty, chaotic MARNIE has been plagued by disturbing sexual thoughts for as long as she can remember. At the end of her rope, Marnie jumps on a bus to London on a mission to change her life and figure out what is wrong with her. Loose and alone in the big city, she tumbles into the lives of eccentric Shereen, sardonic Amber and charismatic Joe.
Marnie hits another dead end when she goes to a sex addict’s meeting but it still doesn’t explain what’s wrong with her. Things start to look up when she finds an unexpected kindred spirit in recovering porn addict, Charlie, and gets closer to Amber and Joe after she talks herself into a job opportunity at Ethel Magazine.
Marnie is determined to make the most of being a single Londoner now she knows what is wrong with her. She jumps into bed with Benji - an earnest, clingy barista - but it has unintended consequences at her work. Meanwhile, Charlie takes a chance at rekindling his relationship with his ex-fiance, Flora.
Marnie and Charlie try to pick themselves back up after hitting rock-bottom. Marnie gets help from an unexpectedly understanding Shereen while Charlie tries to prove he’s up to his old job. Marnie reaches out to Amber only to find herself thrown together with a newly interested Joe. Meanwhile, Amber gets the chance to interview one of her heroes and tries to resist temptation.
Helen, Marnie’s best friend from Scotland, is visiting London and Marnie is terrified of the sexual things that pop into her head when she is around. Encouraged by her new therapy, Marnie tries out trusting her old friend with her secrets and is beyond pleased when Helen seems to understand her problem. Buoyed, Marnie brings her two worlds together at a big dinner party. What could possibly go wrong?
Marnie is shaken to the core when she realises she’s pushed Shereen too far and starts to become paranoid that everyone has turned against her now they know about her condition. As everyone gathers for Amber’s birthday, Marnie becomes consumed by the idea that she isn’t getting better and begins to spiral.
