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.

A divide between the twins widens as they split up with Emiko and Hatcher to find more pages. Calliope tries to connect with Simon. Bree tries a different approach to fencing, forcing Mallory to get out of her head and throw a party.

The vorpal sword is the one thing that can kill an ogre, but Mallory will have to trust Jared to acquire it. They risk it all by following a hag to the Goblin Market. Thimbletack's left to defend Spiderwick.

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.