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.

On his ninth birthday, young Gwyn (Osian Roberts), who lives on a remote hill farm in Wales, receives five strange gifts from his grandmother (Siân Phillips): a piece of seaweed, a yellow scarf, a tin whistle, a metal brooch and a small broken horse. Gwyn offers the brooch to the wind and receives back a tiny silvery spider - Arianwen, the snow spider - confirming that, as his grandmother had already guessed, he has inherited magical powers from his Celtic ancestor Gwydyon, a powerful magician whose exploits are described in the fourth book ("Math Son of Mathonwy") of the Mabinogion. With the help of the snow spider, Gwyn embarks on adventures involving other worlds of snow and silver, as he attempts to solve the five-year-old mystery of his sister Beth's disappearance in a snow storm.

Jac and Rhys are looking forward to enjoying a weekend of playing sport, and getting to see Grandma. There is little chance of fun in the company of adults - or is there?