An angry Thimbletack is out to get Jared, knotting his shoelaces together so he falls down and making the tap water burn like fire. It's dangerous to keep Arthur Spiderwick's Field Guide, but Jared is afraid they'll need it to know how to deal with the magical creatures who live in and about their house. They've never met their old Aunt Lucinda who lives in a nursing home, but since it was her father who wrote the Field Guide and drew its pictures the 3 children decide their aunt is the only one who can help them. Aunt Lucinda has some sad secrets and the children are shocked to find her life depends on magic and tiny flying beings. They also learn she knows how it feels for a father to suddenly leave his family. In hopes of discovering why Arthur Spiderwick disappeared long ago, Mallory, Simon, and Jared meet more magical creatures including a unicorn. Then one of the twins is captured by elves. Will he be trapped in their realm forever? This is the third exciting Spiderwick fantasy.