It turns out the giants control the population of Hydra, a dragon like creature that is creating sinkholes all over Florida. But with the mermaids refusing to return the giants to the shore, the nixie's still missing and the threat of a destroyed Florida drawing closer, the kids have to take matters in their own hands.

Will Nick and Laurie be able to stop the destruction they unwittingly caused? Can a new giant hunter help save the day? Can Arthur Spiderwick's Field Guide help them out of this or are they on their own?