Magnus Chase is beginning to adjust to his new life in Hotel Valhalla. Until a few months ago, Magnus was an orphaned teenager living on the streets of Boston. After dying a heroic death, however, he arrived at the hotel where he learned that all of the Norse gods really exist, and one of them — Frey — is his father. As a result, Magnus gets to spend his afterlife training to fight in a final battle between gods and giants called Ragnarok.
About six weeks after Magnus defeated the Fenris Wolf, Samirah al-Abbas (Sam), the Valkyrie, asks to meet him for coffee. Sam has been busy collecting the souls of dead heroes and keeping up with her homework. Sam explains that they are supposed to meet an informant but gets summoned to collect a soul before he arrives. Magnus waits for the informant who Sam promises he’ll recognize.
Magnus notices a goat in a trench coat and recognizes him as Otis — one of the goats that pulls Thor’s chariot. Otis explains that Thor’s hammer is missing, and the giants of Jotunheim are preparing to attack the mortal world. Without his hammer, Thor can’t stop the invasion, so Magnus must try to find and return it before the giants act. Otis has heard from a source that the hammer might be hidden in the tomb of a powerful undead creature called a wight.
As soon as Otis delivers the information, an unknown assassin kills him with an ax. Magnus tries to heal him using power from Frey but is unsuccessful. Magnus isn’t too worried since Otis is butchered and resurrected daily by his master, Thor. Magnus then chases the assassin who, before escaping, warns Magnus not to go to the wight’s barrow in Provincetown.
Magnus returns to the coffee shop to find Sam. The two then return to Valhalla while she explains that the soul that she left to collect belongs to her brother — another child of Loki. Magnus meets Sam’s brother and his new hallmate, Alex. Alex is a gender-fluid shape-shifter and asks to be referred to with she/her pronouns until she says otherwise.
All of the warriors in Valhalla participate in a daily battle as part of their training for Ragnarok. On this particular day, Magnus and his hallmates, including Alex, are faced with a dragon that’s unlike anything they’ve ever seen. Alex suspects that her father, Loki, sent the dragon since it isn’t able to leave its home at the roots of Yggdrasil — the tree that connects the nine worlds — on its own.
While Magnus is fighting the dragon, he has an out-of-body experience. Loki, the god of evil, used Magnus’ uncle, Randolph, to create a connection through which they can communicate. Loki explains that the only way to avoid a war with the giants is through a marriage alliance, so he has arranged a wedding between Sam and a giant in five days. He instructs Magnus to bring the bride price. Alex notices that Loki is channeling power through Magnus and kills him to break the connection since warriors in Valhalla are automatically resurrected.
After the battle, Alex is welcomed to Valhalla with a feast. Magnus uses this opportunity to talk to Sam about his conversation with Loki. She explains that Loki has arranged a marriage between her and an earth giant named Thrym. To make matters worse, he sent invitations to her grandparents and to her fiancé, Amir, forcing Sam to explain her godly lineage and position as a Valkryie to her mortal family. Sam believes the best way to prevent this marriage is to find and return Thor’s hammer, so they immediately make plans to travel to Provincetown.
Magnus and Sam meet up with Hearth and Blitz — an elf and dwarf that Magnus befriended while living on the street — and they head to Provincetown. Since Blitz is a dwarf, they need his knowledge of earth and stone to find the entrance to the tomb. In the center of the tomb, there is a sarcophagus and mummified men stand in alcoves in the walls. The group decides to open the sarcophagus to look for the hammer, which has the unfortunate effect of waking up both the wight in the sarcophagus and the mummies in the walls.
The four friends are able to defeat the mummies, but Thor’s hammer is not in the tomb. Instead they find a weapon called the Skofnung Sword and realize that they were set up by Otis’ source. Loki and Randolph suddenly appear in the tomb using a portal. Loki explains that the sword will be the bride-price for Sam’s wedding to Thrym. He also reveals that Thrym is the one who stole Thor’s hammer. If Sam marries Thrym, however, and gives the giant the Skofnung Sword and its whetting stone as a bride-price, the hammer will be returned.
At Loki’s request, Randolph grabs the Skofnung Sword, which was still lying on the floor of the tomb, and stabs Blitz with it. Magnus attempts to heal him with the power of Frey, but the wound won’t close. Loki explains that the Skofnung Stone is the only thing that can heal a wound inflicted by the sword. Hearth’s father owns the stone.
Loki leaves them with instructions to bring the sword and the stone to the wedding in four days. Magnus remembers that sunlight will turn dwarves to stone, so he exposes Blitz’s skin to direct sunlight to preserve him until they can find the stone.
Sam must return to her home in Boston, so Magnus and Hearth travel to Alfheim — the world of elves — carrying a petrified Blitz. When they arrive at Hearth’s childhood home, they ask Hearth’s father, Alderman, for the stone, but he refuses, stating that they must give him something in return. The elves respect Frey, so Alderman asks Magnus to be his guest of honor at a few parties to improve his reputation. Hearth, however, must pay his blood-debt before Alderman will give them the stone.
Alderman blames Hearth for the death of his other son, Andiron. When they were children, Andiron and Hearth often played in the forest. A forest beast attacked Andiron. Hearth was a short distance away but did not see or hear the attack, and his brother died. Hearth must earn enough gold to cover the hide of the beast that killed his brother before Alderman will consider his debt paid.
Hearth and Magnus know they will not be able to work off the debt before the date of the wedding, but they hear rumors about a dwarf that hides his treasure in the forest behind Alderman’s house. They manage to steal the treasure and use that to pay the debt. Alderman reluctantly gives them the Skofnung Stone, and they are able to heal Blitz after using running water to unpetrify him. Blitz, Hearth and Magnus then return to the mortal world.
Magnus and his friends are running out of time to find the hammer and prevent Sam’s wedding to Thrym, so they ask a god named Heimdall to help them locate the assassin that warned Magnus against going to the wight’s tomb. They discover that he is a giant named Utgard-Loki. Magnus, Alex, Sam, Hearth and Blitz travel to Jotunheim to find him and discover what he knows. After they best the giants in feats of skill, Utgard-Loki is willing to give them information.
He explains that the wedding is just a cover for Loki’s escape plan and was never about Thor’s hammer. Loki has been imprisoned for centuries and can only appear in the mortal world in a weakened form. The Skofnung Sword is one of the only weapons that could cut his chains, and even it would need to be repeatedly sharpened, which is why he needs the stone as well. If Loki succeeds in escaping, it could trigger Ragnarok, unless he is promptly recaptured. The giant is unable to tell them how to prevent Loki’s plan.
As the five of them prepare to leave Jotunheim, they see a goddess beckon them through a portal. They follow her and arrive in Thor’s home in Asgard. The goddess, Sif, offers to help the heroes in their quest to reclaim the hammer. They decide that Alex will disguise herself as Sam and pretend to go through with the wedding, since Sam has vowed she will not have any part in a marriage alliance with Thrym. Sam and Magnus will accompany Alex and will be in communication with Thor, Hearth, Blitz and some warriors from Valhalla. Thor and the warriors will attack once Thrym retrieves the hammer to bless the ceremony, killing the giants and reclaiming the hammer.
Sam, Magnus and Alex arrive at the wedding as planned, but the giants immediately move the ceremony into the cave where Loki is imprisoned, using earth magic to seal the entrance behind them. The ceremony begins, and Magnus is forced to give the sword to Randolph as the bride-price. Thrym summons the hammer as promised, and Loki commands Randolph to cut him free. Alex, Sam and Magnus attempt to fight the giants on their own but are outmatched. The gods and other warriors are finally able to break into the cave and defeat the giants, but Loki escapes by opening a cavern in the floor of the cave. Magnus tries to save Randolph, but he falls into the cavern as well.
The warriors all return to Hotel Valhalla to celebrate the return of Thor’s hammer, but the hotel manager summons Sam, Alex, Magnus, Hearth and Blitz. He explains that since they all played a role in Loki’s escape, it will be up to them to recapture him. They will have to sail to Scandinavia to find the Eastern Shore where Loki is constructing a ship to sail to Asgard once Ragnarok begins.