My understanding was that Mario and Luigi were working as plumbers working in Brooklyn combating the Shellkreepers or "Koopas" coming out of the pipes when the source of these Koopas, a magical pipe, lead them to the Mushroom Kingdom (much like how the pipe in 3DWorld leads them to the land of the Pixies). The Mushroom Kingdom has a hierarchy of inhabitants ranging from lowly Goombas to Mushroom Retainers to humans such as the various Kings and Princess Peach Toadstool. The lower the class, the more mushroom-like the creature. The Goombas were unhappy with their lot in life and sided with King Koopa when he invaded. The mushroom Kingdom contains various diverse lands, some of which have their own king. There are other lands around, such as the Dinosaur Land that is the home of Yoshis, or the setting of the Gameboy games where Princess Daisy runs the show.
