It could be the perfect summer holiday. Lynn, Marie and Lia have resolved to lie lazily in the sun on the shores of Lake Stechlin every day and do nothing. If only Lynn didn't have nightmares. Night after night, ghostly figures warn of the horror in the depths of the lake and ask the girl for help. When a horse-like monster appears on the shore, Lynn's friends disappear without a trace. Their nightmares seem to come true ...

As if that wasn't enough, the mysterious Daja also upsets Lynn's emotional life, while the search for a blue heart made of crystal brings her closer to mortal danger, but also to an ancient secret.