Tilly, George and Jenny meet up again for their holidays and whilst exploring on the beach they come across a cave with an amazing, radiating crystal inside it. It pulls at them and almost hypnotises them and on their next visit to the cave they bring Mr. Jenkins with them, an expert on crystals. He informs them that the crystal is a Calling Crystal and all four of them are drawn into it. They find themselves in the City of Crystalis amongst the Crystaleans. Jenny transforms into the Great Elder Laia, the leader of the City and Mr. Jenkins is taken prisoner. George and Tilly must mingle with the Crystaleans, communicating with them and learning their ways. They have to find some way of rescuing Mr. Jenkins and also of getting Jenny back to normal, because as Laia she has no recognition of them. They go through many adventures before finally getting to the stage where they might be able to rescue their friend and cousin but nothing is ever as easy as it seems .........