Ista Flit and her friends Nat and Ruby have foiled a dastardly plot by the Hettle family to take control of the town of Shelwich, but Ista still needs to find her missing father. A magical key transports her to the mysterious Glass Island, where a sinister monster haunts the reed-beds, and the local inhabitants are falling under the seductive spell of the marsh spinners. Boldly venturing into the marsh spinner court, Ista and her friends will need all their wits and Tidemagic to resist its magical pull. Can they find Ista's father, or will they be lost forever to the power of the spinners' sinister music?