It's been a year since friends ABE and Pru joined Mister Fox's Fantasy Investigation Bureau - otherwise known as the Unbelievable FIB - to save their hometown from an invasion of Viking gods and giants. Life has been incredibly ordinary ever since. But that's all about to change when the Norse Allfather, Odin, appears with terrible news: Baldur, his favorite son, has been murdered. It's the first step in a fated chain of events that leads to Ragnarok - the end of the world.
"What is the Unbelievable FIB?" That's the question eleven-year-old Prudence Potts discovers on a baffling card no one else in Middleton--except Abe, a new kid with a knack for solving riddles--seems to see. Then a mysterious man asks for ABE and Pru's help investigating mythical beings infiltrating the town, and that's just one of the things Pru finds hard to believe. Soon Pru and ABE discover another world beneath the surface of their quiet town, where Viking gods lurk just out of sight.