Simon, Jared, and Mallory Grace move into the secluded Spiderwick Estate with their mother, Helen, after their parents' separation. Shortly after arriving, Jared discovers a mysterious book, "Arthur Spiderwick's Field Guide to the Fantastical World Around You," hidden in the estate's attic. Despite warnings not to open the book, curiosity gets the better of Jared, and he delves into its pages, uncovering a hidden world of magical creatures that coexist alongside the human world. Various beings such as fairies, goblins, and an old, protective brownie named Thimbletack become intertwined with the Grace family's life.

As Jared and his siblings explore the realm unveiled by the guide, they learn that the book holds powerful secrets that many magical creatures desire, including the malevolent ogre, Mulgarath. Struggling to protect the book and their family, the siblings face numerous challenges that test their unity and courage. In partnership with new and unusual allies, they must decipher the clues left by Arthur Spiderwick to prevent the book from falling into the wrong hands and causing irreversible havoc.