"What is the Unbelievable FIB?" That's the question 11-year-old Prudence Potts discovers on a baffling card no one else in Middleton seems to see, except for ABE, a new kid at school with a knack for solving riddles. Then a mysterious man asks for ABE and Pru's help investigating mythical beings infiltrating the quiet town, and that's just the first of many things Pru finds hard to believe. Soon Pru and ABE discover another world beneath Middleton, and find themselves locked in a battle against a dangerously clever enemy. They must secure the Eye of Odin, source of all knowledge, and the key to stopping a war that could destroy both human and immortal realms.