16-year-old Marnie lives in the idyllic coastal village of Clevedon. Despite being crippled by a childhood exposure to polio, she seems set to follow in her mother's footsteps, and become a 'dipper', escorting fragile female bathers into the sea. Her life is simple and safe. But then she meets Noah. Charming, handsome, son-of-the-local-Lord, Noah. She quickly develops a passion for him - a passion which consumes her. As Marnie's infatuation turns to fixation she starts to lose her grip on reality, and a harrowing and dangerous obsession develops that seems certain to end in tragedy.