Thor's hammer is missing...again. It seems the thunder god has a disturbing habit of misplacing his weapon, which happens to be the mightiest force in the Nine Worlds. But this time the hammer isn't just lost; it has fallen into enemy hands. If Magnus Chase and his friends can't retrieve the hammer quickly, the mortal worlds will be defenseless against an onslaught of giants. Ragnarok, the Norse apocalypse, will begin, and the Nine Worlds will burn. Unfortunately, the only person who can broker a deal for the hammer's return is the gods' worst enemy, Loki, and the price he wants is very high.