Zach and his sister, Rachel, are teenagers whose home is a farm in the Yorkshire Dales of northern England. Their favourite pastime is potholing in the isolated moorlands. A fine spell of weather has exposed a new labyrinth and their exploration awakens the sole survivor of a pioneering probe which landed on Earth aeons before. The alien wishes to pass on his considerable powers to the siblings before his energy dissipates. In so doing, he gives up his existence and the siblings lives are changed forever by the powers they aquire.