The Grace family moves to their ancestral home of Spiderwick, unaware they’ve been manipulated by malevolent forces. 15-year-old Jared realizes something’s in the house with them, yet no one believes him. Simon misses his absentee father and struggles finding his own place. Mallory’s audition with the fencing maestro doesn’t go as planned. Helen re-connects with her eccentric aunt Lucinda.

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! Simon finds a friend in Dr. Brauer’s daughter, Calliope. Jared shows Simon their great-grandfather’s field guide on magical creatures, but all the pages are missing.

Jared grows paranoid of the evil force conspiring to get Spiderwick's Field Guide. Taking Lucinda’s warning to heart, Jared wants to find the pages with Simon alone, but Emiko, Hatcher, and Calliope can’t help joining the adventure. Mallory investigates a mysterious connection. Helen second-guesses her decision to move to Henson but Dr. Brauer tries to persuade her to stay with an enticing offer.

A divide between the twins widens as they split up with Emiko and Hatcher to find more field guide pages. Calliope tries to connect with Simon by sharing her truth. Bree tries a different approach to fencing, forcing Mallory to get out of her head and throw a party. Dr. Brauer and Helen hold off a mob wanting to shut down Meskwaki, igniting a surprising connection. Mulgarath eats another victim.

Simon goes to his father for help. Calliope takes Simon to a friend who turns out to not be so friendly. A frantic Helen finds unexpected support from Dr. Brauer when they discover their children are missing. Jared has to find the dragon page before Mulgarath and has no choice but to join forces with Thimbletack. Mallory challenges Valentina for answers and gets more than she bargained for.

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. Simon’s friendship with Calliope leads him further from his family. Helen and Dorian’s bond strengthens as they face off against Tanner to save Meskwaki. Mulgarath spreads a sleeping sickness.