If you're from another region:

The Plane/Boat you were on arrived at your destination, The beautiful region of Hoenn!

You get off the Boat/Plane, and you go to Mauville city to get an apartment! The room across the hall seems lively, so you decide to knock.

If you're from Hoenn:

You just got an apartment in Mauville city, and you hear laughing in the room across from yours, so you decide to knock.