Stuck in the quaint storybook village of Saint Galdramour for the summer, with his grandmother and her shop of dusty antiques, Jack is homesick and longing for friends until he meets an enthusiastic boy named Finn. Together they investigate the mysterious disappearance of his grandfather Henry, a renowned fairy hunter who after a decade gone suddenly appears only to vanish again. His grandmother fears witches are to blame. While searching for clues Jack has an unusual encounter in the woods and is bestowed with the title of King of the Fairies, however unbeknownst to him, the honour comes with a great many strings attached.
The Grimalkins, a magical race of cats, introduce Jack to the dark and troubled world of fairies, a ruined kingdom where magic is fading and it isn't clear who is to blame. For centuries they have protected humans from the creatures that scurry beneath us, from the perils that linger in the shadows and from forces beyond our perception. However it soon becomes evident that the Grimalkins may have grown too pampered and lazy and aren't up to the challenge of protecting the world from witches anymore.
The devious gloomfairies of the Unhallowed Court, the vengeful forest spirits of the Undergloom, and the great and powerful Oberon himself, all seek to keep Jack from his goal. With time running out Jack must find his grandfather in the land of the fairies before the midsummer solstice is upon them and the witches descend upon the village.