Lark struggles when her family and their friends go on holiday in a lonely caravan site on the Welsh coast for the autumn half term. Her mother is ill, her little sister has stopped speaking and she has fallen out with her best friend. Is a girl in a green dress following her in the fog? Or is her sister playing tricks on her? When a local woman tells her ‘the girl’ comes to take sisters, Lark finds herself fighting to save her family.