Cat Peters will do anything to participate in Darkmont High's student exchange to Sweden, even if it means endless fundraising and striking a bargain with her teachers. Hanging out with friends in a foreign country, traveling without parents, not having a kid sister to keep an eye on it sounds like the trip of a lifetime! And finally, Cat will be free from the constant worry of Grim Hill.But in order to afford the trip Cat must make a major sacrifice: letting her little sister Sookie tag along, which means both of them will be staying with their great-aunt Hildegaard, who has a startling secret of her own. It turns out Cat is not the first fairy fighter in the family, and the skills she's learned battling Grim Hill's evil creatures will come in handy against an ancient Scandinavian threat. The tales of witches, elves and trolls are eerily familiar to Cat, and soon she and her friends are trying to unravel a frightening new mystery.