In the fourth installment of the Spiderwick Chronicles the Grace children continue their battle against the assortment of magical creatures (elves, goblins, ogres, dwarves, phookas) that are trying to get their hands on the Field Guide, an instructional book written by their ancestor Arthur Spiderwick. The Field Guide, which provides detailed information on dealing with magical creatures, was in the Grace childrens' possession until the house brownie Thimbletack stole it from them. As the story begins, twins Jared and Simon, along with their mother, are watching their older sister Mallory in a fencing competition. Although Mallory fences brilliantly, Jared sees an exact replica of himself rooting through Mallory's bag during the fencing match. When Jared confronts his doppelganger he realizes that he is confronting a shape shifting faerie or other magical being. Knowing that faeries detest metal of any kind, Jared pulls out a pocket knife and brandishes it at the faerie. The faerie manages to change shape into the body of a small boy just as the vice principal enters the hallway and sees Jared brandishing a knife at a small child.
Jared's mother is then called into the principal's office and it is apparent that Jared's future at the school is on shaky ground. Moments later Jared and Simon realize that Mallory is missing. When the twins spot Mallory's fencing medal lying in the grass alongside a rock with the words trade etched in the stone, they realize that she has been kidnapped. Immediately, the twins realize that whoever took Mallory wants to trade her for the Field Guide, which they no longer have in their possession.
Feeling in their gut that the stone came from the quarry, the twins set off in pursuit of their sister. Jared wraps up his algebra book and plans to pass it off as the Field Guide in exchange for Mallory's return. Jared is in a very tumultuous state of mind, due to the fact that he overheard his mother on the phone with his father talking about sending him to live with his dad. The boys wait until nightfall and then head to the quarry where they are forced to climb down onto rock ledges to the quarry floor. At the quarry floor the twins discover a glowing stone with a clue written on it: SEEM TO TRICK HEN TOOK PEN. Simon deciphers the clue as KNOCK THREE TIMES TO OPEN. When the twins knock three times the ground shifts beneath them and they are catapulted into an underground chasm where they are met and taken prisoner by a group of dwarves. Jared and Simon are locked in a silvery cage and taken to the dwarf leader, Korting. They are shown a massive metal tree called the Ironwood Tree, which appears to be a symbol of immortality. When the twins are led to the ruler Korting, he advises them that he and the other dwarves are loyal servants of the ogre Mulgarath, who they believe will soon take over the world. As Jared and Simon barter with Korting for Mallory's freedom using the fake Field Guide, they must use all their ingenuity to rescue Mallory and themselves.
The review of this Book prepared by Sandra Calhoune