Sixteen-year-old Magnus Chase has lived on the streets of Boston ever since his mother was killed by wolves two years earlier. He has survived with the help of two other homeless guys, Hearth and Blitz. In all that time, no one has come looking for him—until now.

His estranged relatives, Uncle Frederick and cousin Annabeth Chase, are handing out flyers with his picture on it. Why they’re searching for him now, though, he hasn’t a clue.

To find answers, he breaks into the one place his mother always warned him to steer clear of: the Chase family mansion owned by his Uncle Randolph. He’s poking around Randolph’s office, which is filled with Norse artifacts and maps, when his uncle appears behind him. As if that’s not freaky enough, Randolph tells him that someone is coming to kill him. If he wants to live, he must come with Randolph to the Longfellow Bridge.

Living is something Magnus likes, so even though he doesn’t trust his uncle, he goes along. Things get weirder on the drive: Randolph explains that the Norse gods are real, and that Magnus is the son of Frey, the Norse god of summer. If Magnus wants to survive, he must recover an ancient sword that was lost at sea a thousand years ago. So all in all—just a typical conversation with a long-lost crazy relative. Except . . . Uncle Randolph isn’t crazy. Standing on the bridge, Magnus can feel the sword buried deep in the water below. He summons it and it flies to his hand.

That’s when things get really interesting.

An explosion rocks the bridge. A fiery giant named Surt walks out of the fire and demands the sword. When Magnus refuses, Surt moves in to kill him. Magnus’s friends, Hearth and Blitz, appear and try to stop Surt, but the fire lord is too powerful. He hurls a cannonball of molten pavement into Magnus’s stomach. Then he charges in to wrench the sword from Magnus’s grasp.

But Magnus doesn’t let go. Still clutching the sword, he falls off the Longfellow Bridge into the Charles River, taking the sword and Surt with him. And then he dies.

Sort of.

Moments before he dies, Magnus is swept up by a strange girl on a flying horse. She speeds him to Asgard, the realm of the gods, and a mysterious hotel called Valhalla, where the Norse god Odin’s chosen warriors, the einherjar, come to live when they die. Good news/bad news: Magnus’s room is ready.

While Magnus tries to wrap his brain around the fact that he is dead, the strange flying girl appears. Her name is Samirah al-Abbas, and she’s a Valkyrie, in charge of selecting honorable dead to join Odin’s forces. Come Ragnarok, the Day of Doom, Magnus and the other warriors will battle with the gods against the giants.

The gods are destined to lose—nothing can stop that because, well, fate—but they can delay when Ragnarok occurs by preventing certain events from taking place. One such event is the freeing of Fenris Wolf, a bloodthirsty beast imprisoned by magic ropes on a mystical island, by the fire giant Surt. Surt needs the sword Magnus just found (and immediately lost) because it is the only blade that can cut through those ropes.

It’s up to Magnus to find the sword before Surt does, and then to keep it out of the fire lord’s hands. Luckily, he has help: his buddies Hearthstone, a deaf elf learning the magical power of runes, and Blitzen, a brave and loyal dwarf with a snazzy sense of fashion. They set off for Midgard, realm of the humans, to begin the search. Their quest takes them to different realms on Yggdrasil, the World Tree, from Jotunheim to Vanaheim to Nidavellir. Finally they make it to Lyngvi Island, Fenris Wolf’s prison, where they battle Surt and his army of fire giants.

In the process they discover that an evil being worse than Surt is gaining strength and manipulating others to do his bidding. If Magnus and his friends can’t stop him, the Day of Doom will arrive—and their worlds will be destroyed.