Mertz is a distant, alien world populated by super powered humans instead of anthropomorphic animals . On Mertz, everyone possess incredible powers, except for Ordinary Guy. Comet Guy hails from Mertz.
Possibly before the series takes place, Ordinary Guy was protected by the superheroes of Mertz. In Smarter Than a Speeding Bullet, Comet Guy travels to Earth (which he has mistaken for the planet Flug) to become a champion. In Planet of the Capes, we are fully introduced to Mertz. We learn everyone on this planet is a human and has super powers, with the exception of Ordinary Guy. Ordinary Guy was temporarily replaced by Darkwing Duck. Ordinary Guy renamed himself "Extraordinary Guy", as he created a ray gun that can give strength and suck superpowers from superheroes.