A few months earlier, Magnus Chase and his friends — a group of warriors descended from the Norse gods — were tasked with finding and returning Thor’s hammer. They were unaware, however, that the theft of Thor’s hammer was arranged by Loki, the god of evil, as part of his plan to escape imprisonment. Loki’s escape is one of the events that is prophesied to trigger Ragnarok, the final battle between gods and giants. Unless Magnus and his friends can find Loki and recapture him, the end of the world could occur sooner than expected.
All Magnus and his friends know is that Loki is somewhere on the border between Jotunheim and Niflheim, the worlds of giants and ice, preparing to sail a heavily armed ship into battle. They must find the ship before midsummer, when the ice will recede enough for it to leave harbor, and prevent it from sailing.
Magnus has spent the past few months learning how to sail from his cousin’s boyfriend, Percy Jackson, in preparation for the sea voyage to Jotunheim and Niflheim. Percy is the son of the Greek sea god named Poseidon. After a particularly grueling training session, Magnus and his friend Alex Fierro decide to visit the house he inherited from his late uncle, Randolph. Randolph died helping Loki escape, but left instructions in his will for Magnus to carefully search the papers left behind in the mansion. Magnus hopes that his uncle had a change of heart and left a message that will help them defeat Loki.
Magnus and Alex search the house. They are unable to find anything useful hidden in Randolph’s papers or anywhere else. As they reach the top floor, they are startled to see a wolf attempting to break in through a skylight. Magnus believes that the wolf has been sent to retrieve something, so he suggests letting it inside to help them search.
The wolf shows no interest in Alex and Magnus and immediately grabs a mead horn off of the mantle. They are forced to kill the wolf to retrieve it. Inside the mead horn, they find a small journal in Uncle Randolph’s handwriting. Near the end of the journal, Magnus finds a note addressed to him that implies that something called the Whetstone of Bolverk might help them stop Loki. There is a bit more to the message, but it is written in Old Norse, which Magnus can’t read.
Magnus and Alex return to Hotel Valhalla. In his room, Magnus finds a note explaining that the tides are right to sail toward Jotunheim and Niflheim and that his hallmates, T.J., Mallory and Halfborn, are waiting at the dock. Magnus runs into the hotel manager on his way to meet them and asks him to translate the rest of Randolph’s message. All the manager can tell Magnus is that it says something about mead.
Magnus, T.J., Mallory and Halfborn immediately set sail. Alex goes to find her half-sister, Samirah al-Abbas, who uses her Valkyrie abilities to fly her and Alex to the ship. The crew also plans to meet up with Magnus’ friends Hearthstone and Blitzen, an elf and dwarf that he befriended while all three were homeless near a place called Deer Island Lighthouse. Before they arrive at the island, however, their ship is sucked into a whirlpool by nine water giantesses. The ship seems to be headed to the bottom of the ocean but eventually resurfaces in a giant cauldron of mead.
A giant who introduces himself as Aegir lifts them out of the cauldron. He offers them guest rights, meaning that they will not be harmed and are welcome to share a meal with Aegir and his daughters — the nine water giantesses. The giantesses also captured Hearth and Blitz.
Before the meal can begin, however, one of Aegir’s daughters recognizes Magnus — he had a disagreement with Ran, Aegir’s wife, a few months earlier. She instructed Aegir to kill Magnus if he ever had an opportunity.
Magnus knows, however, that Aegir is particularly proud of his home-brewed mead. He claims that they came to talk to Aegir about mead, convincing the giant to at least wait until after dinner to decide whether he wants to kill them.
During dinner Magnus asks a giantess if she knows anything about Bolverk. She explains that Bolverk is the alias that Odin used to steal Kvasir’s mead and also mentions that Kvasir’s mead is one type of mead that Aegir cannot brew. Sam secretly tells Magnus that she believes they can use the mead to defeat Loki.
Magnus promises Aegir that if he lets them leave unharmed, Magnus will challenge Loki to a flyting (or battle of insults) and will use the mead of Kvasir to give himself an edge. He also promises to bring Aegir a sample of the mead as payment.
Aegir agrees but does not want to anger his wife, so he states that he will leave the room to allow them to escape, but they must battle his nine daughters. The giantesses attack as soon as he leaves, and Magnus quickly realizes that he and his friends will not be able to defeat the giantesses alone. He prays to his father for help. A sea god named Njord, who happens to be Magnus’ grandfather, appears and helps them escape. He transports Magnus and his companions, as well as their boat, out of the giants’ hall.
Njord explains that Kvasir’s mead increases the drinker’s eloquence and is Magnus’ only hope for defeating Loki in a flyting contest. Only a small portion of the mead still exists, and it is jealously guarded by a group of giants. Njord states that Hearth and Blitz will have to go on a scouting mission to find Bolverk’s whetstone, which will help them defeat the mead’s guardians.
Njord tells the rest of the crew to sail to a town in England call Jorvik to learn the location of the mead from a giant. The last bit of advice he gives them is that the palace of Skadi is the only safe place that they will find in Niflheim. Njord then leaves with Hearth and Blitz, promising to transport them back to the boat once they have completed their scouting mission.
Magnus and his remaining friends arrive safely in Jorvik a few days later. Magnus, Alex and T.J. go to find the giant, while the rest of the crew stays behind to guard the ship. Magnus, Alex and T.J. must defeat the giant in battle before he will reveal the location of the mead, but they eventually learn that that the mead is in Flam, a small Norwegian town. They return to the ship and continue their voyage. Before they arrive in Flam, however, Hearth and Blitz return saying that they’ve located the whetstone. Hearth’s father, Alderman, has it in his collection of artifacts.
Hearth’s father blamed Hearth for the death of his other son, Andiron, and forced Hearth to pay a blood debt. The only way Hearth could pay the debt was through stealing a dwarf’s hoard of gold. The hoard included a cursed ring that brings out the worst in whoever wears it. In a few short months, it transformed Alderman into a fire-breathing dragon. Hearth and Blitz located the whetstone in the dragon’s hoard, but they need Magnus’ help retrieving it.
The three friends find a portal to Alfheim, the world of elves, and journey into the woods behind Alderman’s house to find his new lair. Hearth, Blitz and Magnus decide that they cannot leave Alderman in this cursed state. They plan to kill him. Underneath the forest floor is a network of burrows dug by magical creatures called brownies. Magnus hides in one of these burrows. Hearth and Blitz lure the dragon over the opening of the burrow, and Magnus stabs him in the belly. The dragon dies, and they retrieve the whetstone.
They return to the ship and arrive in Flam shortly afterward. The giant in Jorvik said that they would need to take a train to get to the location of the mead, so Mallory, Magnus and Sam board a tourist train heading into the mountains. While they are on the train, Mallory’s godly mother, Frigg, appears to them. She tells them where to disembark from the train and also gives them a magical walnut shell that she says will be able to contain Loki if Magnus defeats him in the flyting. She leaves them, and they arrive at their train stop.
Mallory, Sam and Magnus know that the mead is being kept in a cave in the side of a mountain cliff, but they first have to cross a wheat field that is being harvested by several giants. The harvesters double as guards. The giants immediately spot Mallory, Sam and Magnus. They explain that they cannot take a break from harvesting until the field is completely empty, but their blades are dull and the wheat grows back as quickly as they can cut it down.
Mallory offers to sharpen their blades with the whetstone of Bolverk. They accept and are able to finish harvesting the field but are still determined to kill the three trespassers. Mallory suggests that the giants should decide who gets to keep the whetstone first though. They decide that she should toss it in the air and whoever catches it gets to keep it. As they rush to catch it, however, they all impale each other on their newly sharpened blades. The three friends are able to find the mead in the now unguarded cave.
They return to the ship, and the crew sails toward Niflheim. As they get closer, the ocean begins to freeze, eventually forcing them leave the ship and walk. They narrowly make it to Skadi’s fortress before freezing to death. She allows them to stay the night and gives them directions to Loki’s ship. The friends decide that when they arrive at the ship the next day, the majority of the crew will openly attack Loki’s ship, which is manned by an army of giants and zombies. Magnus will then sneak aboard to find Loki. The rules of combat state that all fighting must cease if the army’s leader accepts a personal challenge.
The next morning Magnus drinks Kvasir’s mead, and the friends travel to Loki’s ship. Everything goes according to plan until Magnus issues the challenge. Loki initially refuses. But the leader of the giants forces him to accept as a matter of honor.
Loki and Magnus begin exchanging insults. Magnus grows physically smaller with each new insult from Loki. He realizes that he has no hope of beating Loki at a battle of insults. Magnus decides that he needs to rely on his strengths and realizes that his greatest strength is his team of friends. Rather than insulting Loki, he begins to boast about his friends. With each boast Loki becomes smaller. Eventually he is small enough to fit into Frigg’s walnut shell and the flyting ends.
The battle only pauses during the challenge, so Magnus and his friends leave the ship as quickly as possible afterward. They deliver Loki over to the gods and return to Hotel Valhalla. Magnus decides to use the mansion that he inherited from his uncle to open a homeless shelter, which Alex helps him run. From then on, they split their time between Valhalla and the shelter.