Norman Comer

The Fifth Doctor, Tegan and Turlough arrive on Sea Base 4, a nuclear warhead station under the sea that has some very nasty neighbours.

Young magician Gywn and his friend Nia have been warned to stay away from Emlyn Llewelyn, the strange boy who claims his mother lives on the moon. And yet, a mysterious magic continues to draw them to him. But why? It's up to Gwyn and Nia to solve the mystery, with the help of Arianwen, the Snow Spider.