Thor's Hammer has disappeared again. The God of Thunder has a troubling habit of misplacing his weapon - the most powerful force in the Nine Worlds. But this time, the hammer is not just lost; it has fallen into the hands of the enemy. If Magnus Chase and his friends cannot retrieve the hammer quickly, the mortal worlds will be defenseless against a raid by giants. Ragnarok will begin. The Nine Worlds will burn. Unfortunately, the only person who can negotiate a deal for the hammer's return is the gods' worst enemy, Loki, and the price he demands is very high.