The first book in the series opens in New England, United States, where nine-year-old twins Jared and Simon and their thirteen-year-old sister Mallory move to the decrepit Spiderwick Estate with their mother, Helen. They discover a secret library hidden in the house, and in the library they find Arthur Spiderwick's Field Guide to the Fantastical World Around You, a book full of information on faeries.
Several days later, while Jared, Simon and Mallory are in Uncle Arthur's study, a small man with a hat appears. The little man, named Thimbletack, is in fact a brownie that has transformed into a boggart when neglected. When a boggart, Thimbletack caused them all much trouble. He commands them to not open and read the Field Guide and leave it alone or suffer a terrible fate.