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.

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