George, Tilly and Jenny meet up for their summer holidays again and whilst exploring on the beach they come across a cave with an amazing, radiating crystal in it. It pulls at them hypnotically and when they retrun to it they bring Mr. Jenkins with them, an expert on crystals. The crystal is a calling crystal and all three are drawn into it, finding 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 mix with the Crystaleans, communicating with them and trying to get Jenny back to normal, because as Laia she has no recognition of them. They go through many adventures before getting to the stage where it might be possible to do this but all is not as easy as they first thought it might be ........