Magnus has been homeless, living on the streets of Boston since his mother died in a fire two years earlier. On his 16th birthday, he learns his uncles — whom his mother had told him to avoid — are looking for him. Curious why they’re suddenly interested in finding him, Magnus breaks into rich Uncle Randolf’s massive house to look for clues.
Randolf is a scholar in Norse history, and his house is filled with historical treasures. Randolf catches Magnus and says the boy is in grave danger. Despite Magnus’ skepticism, he gets in the car with his uncle. As they drive to an old bridge, Uncle Randolf says Magnus is the son of a Norse god.
From the bridge, they can see explosions near Randolf’s house. Randolf urges Magnus to use his mind to pull an old sword from the bottom of the river. To Magnus’ surprise, it works. An inky black villain named Surt appears and demands that Magnus give him the sword.
A battle ensues, in which Surt emits intense heat that begins to melt the pavement. Magnus discovers that he is impervious to the temperature change. He attacks and stabs Surt, and they fall over the bridge together.
Magnus dies and finds himself at the immaculate Hotel Valhalla, where the staff of 1000-year-old Norse men tell him to enjoy his afterlife. Magnus remembers learning that Valhalla is a place for heroes who die in battle. He learns that thousands of dead warriors, known as einherjar, reside in Valhalla. The hotel has 540 doors leading out into nine worlds, and each world is a different layer of reality. He’s given a private suite that reflects everything good in his memories.
Samirah al-Abbas, or Sam, visits Magnus. Magnus remembers her as the girl who carried him to Valhalla. She is a Valkyrie, one who chooses the fate of the slain. She takes him to the massive dining hall, filled with other einherjar. He learns the empty throne at the head table belongs to the illusive god Odin, also known as the All-Father of Valhalla.
During the meal, the newbies’ deeds leading to their deaths are revealed. Sam’s rival Valkyrie, Gunilla, tries to make Sam look bad for choosing Magnus as a hero. Seers read Magnus’ future and determine that he is a son of the god Frey. They also make a cryptic and ominous prophecy. They chant that he is wrongly selected as a hero, and Valhalla can’t contain him. They indicate something will happen in nine days and mention the Sword of Summer unbinding a beast. As Sam is banished for choosing Magnus erroneously, she urges him to find the sword.
Magnus enjoys breakfast in the company of his floor mates the next day. They include a Civil War vet named T.J., a fiery redhead named Mallory, a giant half-troll called X and a Robinson Crusoe look-alike called Halfborn. Like Magnus, X was chosen by Sam, and his presence has met with general disapproval.
The einherjar practice combat training daily while waiting around for Ragnarok, aka Doomsday. That’s when armies of gods and giants will fight a final epic battle. After breakfast, Magnus and his floor mates participate in a bloody scrimmage, with plenty of stabbing, shooting and severed limbs. Everyone comes back to life, but Magnus’ healing and resurrection abilities seem to work more quickly than the other heroes.
Somewhere between death and rebirth, Magnus meets Loki, the god of Evil and Sam’s father. Loki tells him Surt, apparently still alive, wants to hasten Ragnarok. Surt will start by freeing a savage wolf named Fenris, who is so dangerous to the gods that he’s been tied up with special dwarf-made rope on a secluded island. Surt will free Fenris in eight days unless Magnus stops him.
Gunilla takes Magnus on a tour of Valhalla. She shows him Asgard, the amazing realm of the gods, and points out the empty streets. She says the gods have been quiet and inactive for two years, and no one knows why. She believes Sam and the forces of evil are using Magnus to hasten Ragnarok. Gunilla warns him not to choose the wrong side of the battle.
As Magnus tries to determine which side is right, he is visited by his homeless friends Blitz and Hearth. They fall from the tree in his room and reveal they are really a dwarf and an elf, respectively. They’ve been working undercover to protect Magnus on the streets. They take him into the World Tree, where they are able to access the various worlds where the Norse gods dwell.
First, Magnus visits his own world, Midgard, and the funeral home where his mortal body lies. He hopes to find the sword in his casket, but he only succeeds in running into his cousin and childhood friend, Annabeth. She clearly knows something about the family’s connection with gods, and she gives him her phone number in case he needs it.
Magnus and his friends find Sam. Then Blitz and Hearth introduce them to the severed (but still animated) head of a god named Mimir. Mimir says that Surt wants to use Frey’s sword to free Fenris, but that Surt will attempt to unleash the beast with or without the sword. He urges Magnus to get the sword before Surt, then to meet and stop Surt at Fenris’ prison island.
Magnus and Sam take a harrowing sea voyage to retrieve the sword from Ran, the sea goddess. When Magnus and Sam reunite with Blitz and Hearth, they discover Gunilla and Magnus’ old floor mates are after them. Magnus and Blitz escape into the world of the goddess Freya, who is Blitz’s mother and Magnus’ aunt. She says her brother Frey surrendered his sword because of his desperate love for a woman.
The sword, feeling betrayed, would never let Frey use it again. She tells them they must get a replacement rope to hold Fenris, in case Surt is able to free him. Blitz reluctantly takes Magnus to visit a dwarf who is both a fine craftsman and Blitz’s mortal enemy. Sam and Hearth rejoin them. The enemy dwarf challenges Blitz to a craftsmanship contest and sends an angry mob after Magnus’ group when he believes Blitz has cheated. Magnus starts speaking kindly to his sword, which talks back and invites Magnus to call him Jack.
The group escapes from the dwarves by jumping off a cliff into water. They awaken in Thor’s domain, where they rescue the god from a giantess. Thor invites them for dinner, and they ask him to reveal the location of the island where Fenris is bound. He reluctantly admits he has lost his famous hammer. He tells them if they can find and return it, he will give them directions. With the help of a giant horse conjured by Hearth, the group flies to the home of the dead giantess’s family.
They discover the family has Thor’s weapon and Gunilla. Once they’ve killed the rest of the giant family, they free Gunilla. She threatens them, and Sam magically returns her to Valhalla. They discover the giants only had Thor’s backup weapon, an iron staff. But Thor appears and is pleased nonetheless. He promises to send them where they most need to be. That puts Magnus in the presence of Loki’s daughter, Hel, a half-beautiful, half-corpse woman. She tells him she will reunite him with his mother if he will stand down and not fight Surt. He ponders the offer but refuses.
The group sails to Fenris’ island where the wolf taunts and mesmerizes them. Gunilla arrives with Magnus’ floor mates, planning to capture the boy and his crew. When the floor mates side with Magnus and Surt arrives with his fire giants, Gunilla decides to fight with Magnus, too. A bloody battle ensues. Gunilla and two of her Valkyrie lieutenants are killed. With Jack and Sam’s help, Magnus is able to bind Fenris with the strong, new rope. Jack cuts a slit in the fabric between worlds and sends Surt away.
Magnus heals his friends’ wounds. As they sail back to Valhalla, Magnus meets his father for the first time. Frey praises Magnus, and the boy can see why his mother loved Frey. Magnus and his friends explain their disappearance to the dubious assembly at Valhalla. As leaders try to determine the group’s fate, the troll X turns into Odin and reveals he’s been working undercover. He vouches for Magnus and his company and presents them with honors and promotions.
After a funeral for the three slain Valkyries, Magnus goes to visit cousin Annabeth. The adventure ends with them preparing to swap life stories. Magnus doesn’t realize that Annabeth is also a demigod, the daughter of Athena. (She is a main character in the author’s “Percy Jackson” series.) In an epilogue, Loki meets with Randolf to tell him how badly he’s failed by not keeping his nephew in check. The evil god poisons and scars Magnus’ uncle as punishment. Loki contends that there are other ways to hasten Ragnarok, and the fun is just beginning.