Having only narrowly managed to escape the clutches of the faerie world, the Grace children, Jared, Simon and Mallory, return home to find it has been ransacked by the evil creature Mulgarath in his search for Arthur Spiderwick's Field Guide. Not only is the precious guide now missing, so is the children's mother...

With the help of some friendly faces in the magical world, the children now have to work out how to find Mulgarath, the guide and, most importantly, save their mother from the faerie world before it's too late.