Kendra and Seth are tricked into retrieving a statue of the demon Ollach the glutton by Errol, an agent of the Evening Star (the Society). In the process, Seth is bitten by the statue, which curses him such that the demon will eat and grow until it devours and digests Seth. Learning what has transpired, the children are brought to Fablehaven by Vanessa, one of three new characters who are helping the Sorenson's to find Fablehaven's secret artifact. The plan is to find and move the artifact (which combined with the other 4 can open the demon prison) before the Society is able to.

The Sphinx, a powerful agent battling against the Society, has vouched for Vanessa (an expert in mystical creatures), Tanu (the potions master), and Coulter (an expert in magical items). The Sphinx also reveals to Kendra that she is fairykind, which explains her ability to see the in the dark and speak fairy languages. However, it soon becomes clear that one of the 3 is a double agent when Ollach and a Society member are allowed onto the property. After Ollach's arrival, Coulter takes Seth into a cursed valley in the middle of the night in order to try to find where the artifact is hidden. In the valley, they are attacked by a revenant and overcome with terror. At the last minute Coulter gives Seth an item that envelopes him in a protective cocoon, while he is turned into a walking catatonic by the revenant.

While in the protective cocoon, Seth is devoured by Olloch, and eventually purged, fulfilling the curse while allowing him to live. Meanwhile, Grandpa Sorenson sets a trap to discover the spy, and Dale is caught in the act. However, he cannot remember how he got there, and Grandpa Sorenson realizes it is Vanessa, who has the power to control people in their sleep, who is the spy. A fight ensues in which all are incapacitated except for Kendra, who escapes into the woods.

Kendra flees to the pond, where the Nyads send Mendigo after her. A fairy, however, gathers others who change Mendigo so he will now obey Kendra. After meeting up with Seth, they return to the house where Seth steals the key to the artifact. Seth takes a potion of courage and goes off to fight the revenant - barely succeeding. With the revenant dead, Warren, Coulter and Tanu are all freed from their catatonic states. Warren and Kendra head off to the tower with the key in search of the artifact.

Warren and Kendra overcome several traps and make their way to the vault - a cat monster that has nine lives. After killing the monster, it becomes larger, more ferocious, and more deadly with each life. Vanessa and Errol, having followed them, team up with Warren to battle the monster. Errol is killed and Vanessa is incapacitated, but the monster is defeated with the renewed Coulter and Tanu.

Vanessa is imprisoned in the Quite Box (which will restrict her powers) by the Sphinx, who takes the artifact (which had the power to heal all those who were injured) and the prisoner previously locked up on the Quiet Box. However, Kendra discovers a note left by Vanessa that says that the Sphinx is actually the ultimate leader of the Society, and he is just waiting for the perfect moment to betray them all.