Tris Catt leads a double life. He appears to be an ordinary boy but in his fantasy life he is also an alien superhero. The underrunners near his home are an important part of his secret life, secret tunnels known only to him.

Unexpectedly Tris’s imaginary world is invaded by a girl, Winola. She is strange but gutsy, and so good at joining in his invented game that Tris doesn’t take seriously the menacing character who she claims is after her - at least, not until they are caught up in a dangerous situation which threatens both their lives...