Helen and her children, 15-year-old fraternal twins Jared and Simon and their sister Mallory move to their ancestral home, Spiderwick. Jared discovers a boggart and realizes that magical creatures are real! The only one to believe him is his great-aunt Lucinda who implores Jared to find the pages of her father's field guide to magical creatures and protect them from the murderous Ogre, Mulgarath.

When Aunt Lucinda goes missing, Helen leads the search, leaving Mallory in charge of her brothers at Spiderwick. Jared is obsessed with finding the magical creature he saw in the attic, enlisting everyone's help to set a trap.

Held captive at Meskwaki, Jared holds his own against the ogre as Mulgarath pressures him for the dragon page. Mulgarath shares his origin story with Jared, trying to show that they're more alike than he thinks.

Mulgarath's plans are coming to fruition as townsfolk either succumb to the sleeping sickness or become the ogre's servants. The gang launches a daring prison raid to rescue Jared. The Grace family must reunite to defeat Mulgarath.