We rejoin our favorite blue-haired heroine Hilda to find her caught up in a crisis in Trolberg. Ships are vanishing from the harbor and no one knows why. Could the answer lie out on Cauldron Island? Meanwhile, David suspects wild weather predictor Victoria Van Gale is up to no good, as she moves into the old windmill.
Hilda, Frida and David are in the Library to do research for their attempt to earn the Birding Badge. When they spot a rare scarlet-capped warbler fly by the window, David chases after it to take a photo, while Hilda and Frida first return the books they were reading. They notice the librarian is gone, and when they try to find her, they discover a series of hidden rooms that eventually bring them to the legendary Witches Tower. They find the librarian there, and realize she is a witch. The girls learn the Librarian is in a hurry to retrieve the book The Skeleton Whisperer from Matilda Pilqvist, which was checked out by her almost 30 years ago. If she doesn’t get it back before the end of the 30 years, she will be cast into the void of no return as a punishment. Hilda and Frida offer to help the librarian navigate Matilda’s maze, which is full of traps and puzzles. Frida in particular proves quite skilled at solving the puzzles, and Kaisa realizes she has the potential to become a witch herself. When they reach Matilda, she offers to become Frida’s teacher.
Meanwhile, David follows the warbler out of the city, and to an old windmill. Here, he encounters Victoria van Gale, whom he and Hilda met before. She claims to have changed since their last adventure (in which she kidnapped a baby weather spirit for her experiments). She made the windmill a shelter for animals. When Hilda and Frida are done with their adventure, they also come to the windmill. Hilda is willing to give Victoria the benefit of the doubt, but David is skeptical about her change of hearth. He offers to help her with repairing the windmill, so he can spy on her.
That night, in her home, Hilda sees a news report about a boat having been attacked and sunk in the harbor; the last one in a series of similar attacks. The news anchor interviews Erik Ahlberg, who naturally blames the sinking on Trolls. He is interrupted by a fisherman, who claims the culprit to be the Lindworm from Cauldron Island. The following day, Hilda goes to the harbor to see if she can figure out what sinks the ships. Instead, she runs into Wood Man, who just won a boat named the Nautilus in a game of Sea Bones. He intends to use it to find the Draugen. Hilda decides to come with him in hopes that the Draugen know more about who or what is sinking those ships. They find the Draugen ship, and Hilda quickly gets along with their captain. Unfortunately, it turns out Wood Man only wanted to find them in order to steal a magic coral sextant for his home. Angered, the captain decides to feed her visitors to the Salt-Lions and thus make them into Draugen. Her first mate however points out a loophole that captives of the Draugen can earn their freedom by defeating the Draugen Ship in a race to the shore. At first, the Draugen Ship easily stays ahead of the Nautilus. Fortunately, Hilda spots a weather spirit, and tells him some jokes (all at Wood Man’s expanse). The Weather Spirits laughter causes a strong enough wind to help the Nautilus win the race. Defeated, the Draugen retreat to open water.
While Hilda has her adventure with the Draugen, David is helping Victoria at the windmill. After repairing the sails (during which he falls off the mill, but a bush breaks his fall), he notices something moving. When he investigates, he finds a Nisse-like mannequin, which turns out to be alive and attacks him. Victoria saves David and explains the Nisse is her assistant; she built him herself.
With the Draugen gone, Hilda joins up with Frida, who has been tasked by Tildy to practice psychobiokinesis (forming a mental connection with a living thing), and David, who tells them about his discoveries. Hilda doesn’t see much harm in the fact that Victoria created her own Nisse. When Victoria hears that Hilda has visited Nowhere Space with Tontu’s help, she wants to know everything about it. At lunch time, a news report on the radio informs that another ship has sunk near Cauldron Island, and that Erik Ahlberg is planning an attack on the island to capture the Lindworm, whom he now believes to be the cause. Hilda, who knows the Lindworm is innocent, decides to go warn her. Frida comes along, while David remains in the windmill.
As expected, Erik Ahlberg refuses to listen to Hilda. Fortunately, Wood Man and his boat are still in the harbor, so Hilda convinces him to take her and Frida to Cauldron Island. The Lindworm has no intention to leave her island however, and prepares to fight Ahlberg and his forces. When the girls return to the boat, they discover the Nautilus is gone. They spot it in the distance, where it is suddenly dragged below the surface. A passing Water Spirit is willing to help Frida and Hilda reach the place where the boat sank. There, they only find Wood Man’s head, separated from his body. He claims that the thing that attacked his ship has tentacles. Also; he didn’t abandon the girls, he was trying to escape the creature. The Water Spirit gives Hilda, Frida and Wood Man bubble helmets so they can go underwater to look for Wood Man’s body. Once underwater, the girls discover a graveyard of sunken pirate ships, and then an underwater bell. It has been rigged to ring regularly, no doubt on Erik Ahlberg’s orders. After some searching, the girls find a strange, octopus-like creature. It is too small to be the creature that sank the Nautilus, but Frida fears this one is just a baby, and the much bigger mum is behind all the ships sinking.
At the windmill, David, with help of Victoria’s nisse, is filling up cracks in the wall. They only skip a big one that Victoria wants to remain open for some reason. After work is done, David checks out Victoria’s bookcase, and comes across a book about necromancy. Startled, David investigates the case further and finds schematics for some sort of machine. It seems Victoria is planning something involving Nowhere Space. The nisse does not appreciate David snooping around, and ties him up to prevent him from warning anyone. After the Nisse leaves, a rabbit enters the mill, and Alfur (who had been away to a grammar conference in the Northern Counties) is riding on his back. Johanna told him that he might find Hilda here in the mill. David quickly tells Alfur about his discovery, and asks him to go get Frida and Hilda.
In the fjord, Hilda and Frida find the mother of the octopus-creature; a gargantuan Kraken whose shell forms Cauldron Island. It has been asleep here for centuries, and according to Wood Man should still be asleep. The underwater bell woke it up. Then Erik Ahlberg arrives with his fleet. As they attack the Lindworm, Hilda tries to warn both sides about the Island’s true nature. Her warning comes too late; as the Lindworm dodges some harpoons fired at her, they pierce the Kraken’s shell and enrage the beast. The Kraken emerges and attacks the fleet. She wrecks several ships, and feeds them to her children. Hilda realizes the Kraken was only attacking boats to feed it’s young. Frida tries psychobiokinesis on the Kraken, and succeeds in telling the beast about the ship graveyard they found. It works; the Kraken turns her attention to the sunken ships and leaves the last remaining boat of Erik’s fleet alone. She also returns Wood Mans’ body. The safety patrol retreats, and the Lindworm gives Hilda, Frida and Woodman a ride back to shore.
Once on land, the three are found by Alfur, who informs them about David’s discovery. At the windmill Victoria learns from her Nisse that David discovered her plan. She claims that she wants to access Nowhere Space so humans can build houses there and no longer have to sacrifice nature for more living space. She straps her Nisse into the machine she has been building, and powers it up. Hilda, Frida and Alfur meanwhile hurry to Hilda’s house and tell Tontu about Victoria’s plan. Tontu uses nowhere space to transport all of them to the windmill. Along the way, Tontu theorizes that all to Trolberg could get sucked into nowhere space if Victoria’s plan succeeds. When they enter the windmill, Victoria has opened a breach to Nowhere Space using the large crack in the wall, and steps in. Hilda and Tontu attempt to follow her, but Victoria’s Nisse attacks them. Frida an twig fight the Nisse, while Hilda and Tontu enter Nowhere Space. Because the nisse left the machine, it begins to malfunction. Hilda fails to convince Victoria to come with her. As the machine falls apart and portal begins to close, Hilda and Tontu evacutate Nowhere Space, while Frida throws the head of the Nisse, which is now stripped to just a skull, into the portal. The portal then closes with Victoria still inside Nowhere Space.
As the kids stay in the windmill a little longer because it has started raining, Erik Ahlberg and Gerda Gustav suddenly enter the mill to investigate the incident. Erik of course blames a troll for the whole mess, and refuses to believe Hilda’s explanation about Victoria’s plan. As he leaves to chase this non-existent troll, Gerda notices the blueprints of the machine lying on the floor, and realizes the kids told the truth. Neverthelesss, she leaves with Erik. When they are gone, the scarlet-capped warbler from the start of the story flies into the mill, allowing the three friends to finally photograph it.
- Johanna (Called “Mum” in this story)
- Trolberg Safety Patrol
- Matilda Pilqvist
- Kaisa (called “The Librarian” in this story)
- Linn Jacobsen
- Victoria Van Gale
The following characters/creatures are mentioned, but make no appearance:
- Void of No Return
- The Committee of Three (called Council of High Witches in this story)
- Emil Gammelplassen
- Illus (as "Bobblehead Mountain" when Hilda recalls how that mountain turned out to be a giant)
- Peter Ostenfeld
The book combines characters and plot elements from the episodes "Chapter 2: The Draugen", "Chapter 3: The Witch", "Chapter 5: The Windmill", and "Chapter 7: The Beast of Cauldron Island". Like in the previous book, Hilda’s adventure from "Chapter 10: The Storm" is briefly mentioned but not shown in detail.
Like the previous book, the novel rearranges the order in which these events happen compared to the animated series, so the events make up a single story rather than individual adventures. It also takes several other liberties with the episodes it adapts. Included:
- Hilda is somehow aware of the secret room with the spell books in the Library, even though the episode in which she discovered this room was never adapted into a novel.
- The part of the book that adapts "Chapter 3: The Witch" omits the climax of that episode in which Kaisa, Hilda and Frida fight the Void of No Return. It is also not stated in the book if Kaisa and Matilda share a history.
- The book mentions that Johanna has a grudge against Victoria van Gale due to her actions in "Chapter 10: The Storm". The animated series never reveals if Johanna holds such a grudge.
- In the series, Hilda intentionally angers a weather spirit in order to create a storm so the Draugen will appear. Here, she doesn’t have to resort to this.
- While Wood Man’s reason for wanting to find the Draugen remains the same, in the series Hilda wanted to find them in an attempt to help them reunite with their families on land.
- In the series, the Draugen continue to chase Hilda and Wood Man on land, until Hilda lures them to the cemetery and reunites them with the ghosts of their families there.
- In the animated series, David and Alfur come with Frida and Hilda to warn the Lindworm and find the real cause of the sinking ships. Also, Wood Man’s ship got wrecked by the Kraken before Hilda and friends went to Cauldron Island.
- The Water Spirit has a larger role in the animated series; there, she is with Frida, Hilda and Wood Man for most of the adventure, letting them ride inside her body. Here, she only gives them bubble helmets and then departs.
- Since the plot of Hilda getting grounded was already adapted into the previous book (with the reason why she got grounded being changed to Johanna discovering Hilda helped to sabotage the mechanical bellringers), Hilda’s adventure with the Kraken does not get her in trouble with her mother in this story.
- The book omits a subplot in which Alfur and David get the Bragga Family to fight the Lindworm so she can have her battle and thus leave the island before Ahlberg arrives without having to break her code.
- The animated series never mentions if people got injured or even killed in the Kraken’s attacks on ships. The book however makes no secret about it that there have been casualties.
- In the series, while David is the one that discovers Victoria’s plan, he does so at his own home, and Frida is the one that ends up getting captured by Victoria to prevent her from interfering with her plan.
- In the series, Erik and Gerda don’t investigate the aftermath of Victoria’s experiment, and nothing indicates the Safety Patrol even noticed anything.