As the final days of 1928 approached, Arthur Spiderwick meticulously compiled his notes and sketches into an extensive field guide, chronicling his encounters with the enchanting realm of fairies. However, his excitement was tinged with apprehension, aware that the malevolent Mulgarath, a shapeshifting ogre, coveted the secrets contained within the Book for wicked purposes. To protect his life's work, Spiderwick concealed it from those who would misuse its power.
Fast forward eighty years, and Helen Grace (played by Sarah Bolger), a recently divorced woman, moves her family into the faded splendor of the long-deserted Spiderwick estate, eager to begin anew alongside her children: Mallory, Jared, and Simon. Tensions quickly rise among the twins as Jared grapples with unresolved feelings toward their father. However, a chance discovery of a concealed dumbwaiter catapults Jared into a world brimming with wonder, right into the study where Spiderwick once immersed himself in his fascinating research.
As Jared navigates the bizarre and mystical treasures hidden within the estate's walls, he seeks refuge in the ancient texts and artifacts, while his family remains skeptical of his fantastical claims of magical creatures. Undaunted, he persists in his curiosity, leading him to uncover life-altering secrets locked within the estate.
The following day, Jared draws the attention of an unexpected ally, Thimbletack, a brownie angered by his careless disclosures. Offering a peace offering of crackers and honey, Jared manages to placate the irritable sprite, who grants him a special holed stone, allowing Jared to see fairies normally hidden from human eyes. Thimbletack also imparts wisdom about the protective mushroom circle surrounding their home, emphasizing its importance.
However, the tranquility is shattered when Jared's innocent world crumbles upon the kidnapping of Simon by crafty goblins led by the sly Redcap. Simon finds himself captive under the guise of an elderly man at a remote goblin camp. Undaunted, Jared infiltrates the campsite and meets Hogsqueal, a fearsome hobgoblin, who gifts him the ability to perceive fairies without the stone, fueling his resolve.
Once Simon is rescued, the twins team up to reclaim their stolen book and seek vengeance against the dark forces that have wrought havoc upon their lives. As chaos ensues, the family retreats to the safety of their home, where Mallory arms herself with fencing sabre, ready to defend against the advancing goblins. In this tumultuous battle, Jared and Simon discover that their bond has intensified, standing firm as they prepare to confront the monstrous challenges ahead, fueled by their newfound understanding of this magic-drenched world.
Driven by desperation, the children seek guidance from their great-aunt Lucinda Spiderwick, Arthur's daughter. While Simon cleverly distracts the lurking goblins, Mallory and Jared make a brave escape through an underground route. Their narrow escape from a troll sent by Redcap becomes a harrowing memory, as they narrowly evade danger when the creature is struck down by an oncoming truck. They soon find Lucinda in her residence at a psychiatric facility, where she reveals that they must track down Arthur and solicit his help to destroy the cursed book that has remained in the grasp of the sylphs for eight long decades. Just as she imparts this critical knowledge, Redcap and his goblin minions launch a sudden assault through the hospital windows, managing to snatch several precious pages from the ancient manuscript before retreating. It becomes evident that the theft has delighted Mulgarath, as the pages hold the secrets to dismantling the protective circle.
Meanwhile, Helen's skepticism ignites a fierce argument with Jared during their return home, culminating in his heartfelt outburst expressing that he no longer wants to live under her roof. Later, Hogsqueal, acquainted with Mulgarath's sinister designs, shares vital information with the children. Driven by newfound urgency, Jared, Simon, and Mallory consult the book, successfully summoning a majestic griffin that whisks them away to the enchanting realm of the sylphs. There, they reunite with Arthur, who, despite his long imprisonment, appears strikingly youthful. However, their meeting is short-lived, as Jared uncovers that Thimbletack has cleverly swapped the crucial pages, leaving them with mere replicas of the sacred text. Arthur's relief quickly shifts to despair upon realizing that Mulgarath now possesses the means to breach the protective circle.
As the family braced for impending doom, Helen begins to accept the truth, fueled by her children's resolute testimonies. Meanwhile, the goblins successfully complete their nefarious plot, releasing a potion that shatters the magical barrier as the full moon ascends to its peak. The family must mobilize to protect themselves against a swarm of goblins armed with kitchen knives and homemade salt-and-tomato-sauce bombs, deadly to these mischievous adversaries. After a thrilling clash, the goblins are ultimately vanquished in a spectacular explosion of tomato sauce. In the aftermath, an unexpected figure appears: Richard, initially presumed to be the children's father, is revealed to be Mulgarath in disguise. With quick thinking and cunning, Jared outsmarts the imposter, sending him tumbling off the roof. As the dust settles, Mulgarath morphs into a raven in a desperate attempt to seize the Book, only to be devoured by Hogsqueal, effectively ending his wicked schemes. Following this turmoil, a long-awaited reconciliation between Jared and Helen finally transpires.
As the weeks progress, the enigmatic Lucinda (portrayed by David Strathairn) joins her newfound adoptive family for an emotional visit back home. Their reunion with Arthur, alongside his ethereal companions, the sylphs, is tinged with bittersweetness, for Arthur's continued presence carries a grave consequence: his essence would fade into nonexistence. In a poignant moment, Lucinda, yearning to reclaim her lost youth, persuades Arthur to allow her to travel with him on this transcendent journey. As if responding to her heartfelt plea, the sylphs gently transform her back into her childhood self. Witnessing this touching scene, the Grace family is left with lingering emotions as they bid farewell to Arthur and Lucinda, whisked away by the sylphs, leaving behind a profound sense of wonder and yearning.
