When Horla's best friend Carar moves to another town, she wishes for a new friend, one that comes from a parallel world to Ivarnio, a parallel world where she becomes invisible and has special powers... A parallel world called Earth.
On Earth, Lucy sits sad and lonely in her bedroom, having waved off her own best friend Eleanor for a new life in Liverpool. Suddenly, Horla's face appears in Lucy's dressing table mirror, and quickly a unique friendship is formed.
Getting up to all kinds of mischief that baffles children and adults alike, together they are the perfect pair. That is, until, Eleanor says she misses Lucy terribly and begs to see her. Soaring up into the sky, Horla and Lucy set off on an adventure to meet her, and when Horla tries to help her make new friends, Eleanor is set the challenge of carrying out a very dangerous rescue; a rescue no other child would ever dare to attempt...