Jaelah and Aydan live deep underground. Their half-light world relies on light-filled crystals for life and light. But the life-lines seem to be running dry – and with this comes the promise of death. The dimming of the world gives Jaelah the chance she's been looking for. Dragging twin brother Aydan behind her, she sets off to seek answers in the corners of the world. The twins must venture far up the tunnels to solve the mystery of what is happening to the magical life-lines and crystals. But there they meet the prince of the surface dwellers, who is lying injured beside the life-giving river. He begs them to take him to the surface – a place the twins have never been before, and where danger awaits them.