Rex the Brave Dog goes down for a moment to the basement in the den of his friend Moles the Mole and literally "the ground swallows him up". The worried commander immediately sets off on his trail and soon, through a labyrinth of corridors and tunnels, he reaches an underground secret city of moles. It turns out that Rex has been captured, imprisoned and sentenced for spying for the Rats, and this, as it soon turns out, is just the tip of the iceberg.