Caitlin and sister Rhona full prey to a Kelpie inhabiting a famous Scottish waterfall. Transformed by the evil horse spirit in to Mari, the fifteen-year-old Victorian Glaswegian orphan in the book she's reading, Caitlin is taken to a fairy-tale land where she believes she's the true princess. But the Kelpie has other plans for her. Can Rhona save her sister? And what about the woodcutter's son?