3rd Gen Mystery Event

I said Mystery Event Is Exciting to this guy in petalburg and he says im part of the Mystery Event club, And now when i turn on their is an option for mystery event. What is it?

Well you got to have two GBAs, one E-reader, that card you got that came with the game, and a link cable.
Put your game in one of the GBAs and the E-reader in the other then link up turn the GBAs on, and the one with the game click the Mystery Event, and the one with the E-reader scan the card. The do what it says, if you did it all right then you will be able to battle in that house with the locked door.

Well, a more generalized explanation is that Mystery Event allows your game to communicate with other games and other media. the E-card Reader does do the battle E-cards, but it also does the Eon Ticket E-card in a similar manner. In addition, it could be used to connect to another GBA cart in the future or maybe a GCN game. So, to sum it up, it enables your game to "listen" for communication from an external device.

Ok, Umm, Whats nan e-reader and what card do you mean?

Plus i do have the eon ticket but i have no idea what to do with it.

The E-Card Redaer is a cartridge you put in your Gameboy Advance to swipe these "E-cards". You then can either play minigames, or use a link cable to transfer data to other game. The cards have dot codes on them that holds the data.

http://nintendo.com/systems/gba/gba_pop.jsp?item=ereader

You can fight Lady Astrid and Gent Nils behind the locked door in Mossdeep.