Simon drills tunnels for his underground community and his brother Kamina is a "troublemaker".  He is jailed for trying to reach the surface which he is convinces exists because he once saw it with his father.  Simon finds a huge face while digging and gets Kamina out to see it.  An earthquake happens and a huge robot like creature falls through to the city followed by a girl named Yoko.  Simon leads Kamina and Yoko to the face which reacts to a key he found earlier and comes to life.  They escape in the craft and take out the fallen robot in the process and they wind up on the surface of their world.