Ioan is a small, quiet and shy ten-year-old who, not surprisingly, everyone, including his mother and stepfather, calls Mouse. The only thing about Mouse which made him different from most other quiet and shy 10-year-olds was that he had a very real and vivid imagination. Most children daydream from time to time, especially during boring lessons, but Mouse 'lived' in his dreams, the real world was rarely visited! He was forever fantasising about being some kind of superhero - Agent Mouse. Mouse is still coming to terms with the tragic loss of his real father. His alter ego - Agent Mouse - helps him to cope when the family move to live in Conway and he gets on the wrong side of, Biston, the school bully. But a school trip to scale the majestic heights of Snowdon has unexpected consequences and Mouse's life will never be quite the same again.