Kiran, Jay and Robyn have been best mates for years, hanging out together in their idyllic hometown on the Cornish coast. Now that they've turned sixteen, they have one last summer before they have to get serious about the rest of their lives. But thier lives change for ever when Kiran attempts to save a stranger from drowning and fails. From that moment on he is obsessed with the girl whose body is never found. Imagining that he can see her everywhere, shunning the company of Robyn, his devoted girlfriend, and testing his friendship with Jay, whose loyalty is shadowed by his growing feelings for the vulnerable Robyn. Robyn is blindly determined to help Kiran over his apparent 'breakdown', but soon both she and Jay are having their doubts as to whether the ghost girl, the Merrow, as they call her, exists after all, and all three are drawn into a sinister and nightmarish search for the truth...