Tired of spending Christmas in their cramped apartment, the Olsson family rents a secluded country house for the holidays. But when they arrive, they discover they've been given the keys to the mysterious Greveholm Castle instead.

As the family settles into their unexpected new home, the children eagerly explore the castle's endless corridors and forgotten rooms. Meanwhile, two ghostly residents are less than thrilled about the unexpected company and begin plotting how to reclaim their peaceful home.

Lillan's new voltmeter picks up something impossible, confirming her suspicions that Greveholm is haunted. While Jean and Staffan step up their efforts to frighten the family away, the children become even more determined to uncover the truth behind the castle's strange inhabitants.

After discovering that the ghosts are real, Ivar and Lillan keep the secret to themselves rather than risk leaving the castle. Armed with homemade ghost-hunting equipment, they begin investigating the spirits, while Melitta recruits an unlikely ally to help prove that Greveholm is haunted.

Jean and Staffan grow increasingly frustrated as their attempts to scare the Olssons continue to backfire. While the children search for more clues about the castle's mysteries, the ghosts realize this family may be unlike anyone they've encountered before.

Determined to prove that the ghosts are real, Ivar devises a clever trap with Lillan's help. While Leif unknowingly begins assembling the mysterious robot parts found in the castle, Jean and Staffan step up their efforts to frighten the family. But after an unexpected encounter with the children's computer, the ghosts find themselves caught in a trap they never saw coming.

After accidentally trapping Jean and Staffan inside the family computer, Ivar and Lillan finally convince the ghosts that they want to be friends rather than enemies. As the spirits begin sharing stories about their past, Lillan continues rebuilding a mysterious robot from scattered parts—much to Jean's alarm.

Måns tries yet another unconventional way of capturing the ghosts, while Lillan gets closer than ever to restoring the mysterious robot. Jean resorts to an old trick to stop her progress, but his plan doesn't quite go as intended. Meanwhile, Ivar sends an unusual Christmas wish list through the castle's computer.

A ghost hunt takes an unexpected turn when Måns captures an unusual visitor instead. Acting on Ivar's Christmas wish list, the mysterious guest delivers something that could finally bring Lillan's robot to life—but the experiment doesn't go entirely according to plan.

On Ivar's birthday, an unwelcome visit from Grandma Anna-Britta leads the family to a forgotten piano hidden within the castle. A centuries-old melody sets off a series of extraordinary events, and an unexpected discovery on the sheet music points Ivar and Lillan toward Greveholm's greatest mystery.

Following the clues hidden within the old menuet, Ivar and Lillan set out in search of Greveholm's legendary treasure. Their hunt leads them beneath the castle, where a remarkable discovery suggests the treasure can only be found in a very unusual way. But someone else has their eyes on the map.

When a valuable map accidentally falls into the wrong hands, two bungling thieves decide to break into Greveholm under the cover of darkness. At the same time, Melitta and Måns launch another ghost hunt during the witching hour, unaware that several midnight visitors are about to cross paths inside the castle.

Lucia morning brings unexpected guests to Greveholm as two disguised thieves blend into the family's celebrations. With help from some unlikely allies, they finally locate the legendary treasure—but Ivar and Lillan quickly realize what's happening and set off on a daring pursuit.

The chase continues as Ivar and Måns race to stop the thieves from escaping with Greveholm's treasure. Once the mysterious chest is finally recovered, the children discover that its contents are far stranger than they ever imagined, raising even more questions about the castle's secrets.

As Jean's missing sword begins changing more than just his mood, Ivar reluctantly agrees to help another ghost in need. Meanwhile, Lillan experiments with the castle's mysterious treasure and uncovers an astonishing property that turns an ordinary afternoon into a series of increasingly unbelievable discoveries.

With Måns busy at his new job, Melitta sets out to capture the ghosts on film by herself. Instead, she discovers that Jean and Staffan are far friendlier than she ever imagined. Meanwhile, Ivar and Lillan finally uncover the true purpose of the mysterious floating stone, leading to a breakthrough for the unfinished robot.

A hidden passage leads Ivar and Lillan to a long-forgotten room deep within Greveholm, where they uncover new clues about the castle's past. Pressed for answers, Jean and Staffan finally reveal the tragic story of how they became ghosts—but some parts of the mystery remain untold.

Determined to uncover the truth behind the castle's greatest secret, Ivar and Lillan send Sprak into the hidden chamber to investigate. But what the robot discovers raises even more questions, prompting Ivar to take matters into his own hands.

When Jean and Staffan suddenly disappear, Ivar sets off on a dangerous mission into Greveholm's hidden tower. As he comes face to face with the castle's greatest threat, his courage is put to the test in a daring rescue that brings the mystery closer to its conclusion.

The children begin preparing for a final confrontation as they search for a way to reach the imprisoned princess. After a series of unusual experiments around the castle, an everyday Christmas drink inspires Lillan with an unexpected plan. Meanwhile, an old acquaintance returns with one last urgent warning.

With Greveholm's fate hanging in the balance, the children put their carefully prepared plan into action. But when their traps fail and everything seems lost, unexpected allies return to help in one final showdown that will decide the future of the castle.

After years of mystery, the princess is finally free—but she proves to be unlike anyone the Olsson family expected. As she reveals the truth about herself and her extraordinary abilities, even the adults begin to question everything they thought they knew about Greveholm.

As Christmas Eve approaches, Astrid and Leif finally accept that Greveholm really is haunted. While the family grows closer to Jean and Staffan than ever before, a familiar visitor returns with an urgent warning that could determine whether Christmas arrives on time.

Christmas Eve has finally arrived, but the Olssons still have one last secret to keep from Grandma Anna-Britta. As friends old and new gather to celebrate, Greveholm's final surprises unfold, bringing the family's Christmas to a magical conclusion.