Lark needs a break. Her mother is ill, her sister has stopped speaking and she's fallen out with her best friend. When her family arrives at a caravan site on the windy Welsh coast, she and her little sister rush off to explore. But soon this freedom get frightening. Is there a figure in the fog? What happened at the ruined house in the woods? Why is her sister so fascinated by a girl in a green dress that no one else can see? As the storms get wilder and events get stranger, Lark must face a long-buried secret to try to save her family.