Nelly and her family are off for a really scary weekend, visiting a genuine haunted castle. At least, that’s what the newspaper advert promises. Lena-Sleva asks Nelly to try out a new invention that Uncle Hannibal has come up with. It’s a ghost detector, and it’s used to hunt for ghosts and ghouls. Because, according to Nelly’s old teacher, ghosts have become very rare these days, and nobody really knows where they’ve gone.