After a narrow escape from the dwarf quarry, the Grace brothers hatch a plan to retrieve the Field Notebook, but when they return to Spiderwick Manor they discover that something even more precious than the Notebook is also missing: their mother! With the help of the elves Thimbletack, Hogsqueal, and Byron, the Grace brothers set out on their latest adventure: defeat Mulgarath and his goblin army before it's too late.