Eleven-year-old Nicholas Vargas only thinks his life has been turned upside down after his developer father remarries and moves his new wife and daughter into the soon-to-be completed Mangrove Hollow. But an "expedition" to a nearby lake turns up a little nixie with a giant problem--the huge, lumbering, fire-breathing variety--and it's up to Nick, his stepsister Laurie, and his big brother, Julian (plus a familiar face from the original Spiderwick...

When the Grace children go to stay at their Great Aunt Lucinda's worn Victorian house, they discover a field guide to fairies and other creatures and begin to have some unusual experiences.

Nick, Julie, Laurie, and their friends try to prevent a hydra from destroying Florida.