Tilly, George, and Jenny meet up once again for their summer holidays and whilst exploring on the beach they come across a cave containing an amazing, radiating crystal. It pulls at them, almost hypnotically, and it is with great difficulty that they manage to get away. When they next return to the cave they bring Mr. Jenkins with them, an expert on crystals. He tells them that the crystal is a Calling Crystal and somehow they find themselves drawn into it and into the City of Crystalis amongst the Crystaleans. Jenny transforms into the Great Elder Laia and Mr. Jenkins is taken prisoner. George and Tilly must socialise with the Crystaleans, communicating with them and trying to get Jenny returned to them as their cousin, 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 simple as it seems .......