in the red yellow and blue that would be it, but in firered and leafgreen, you need the national dex and beat team rocket on five island and get both the ruby and saffire gems on the islands so you can unlock the stronger elite four with new pokemon, then once u beat that, u dont got to fight them again or notin but thats when u know u can go in that cave.
P.S. to get the national dex, you need to catch 60 pokemon in your regular pokedex.

talk to him and he'll realize that your the new Pokemon league champion then he'll move

go to 4 island then go to the house beside the pokemart...

1 u need to become the champ.
2 u need the nax.
3 need a pok. that knos surf/rock smash/ strength.
4 U MUST HAVE A MASTERBALL.
5 get players guide
6 look in players guide
7 it will show u how to get to mew2

complete celio network
(a good person explain how to complete (who got rate thumbs up))
and...
that who get a thum down....
why must need master ball
I use untra ball
if you got luck,
you can catch it in FIVE turn!
i catch it in five turn...

if you have a tri-p*** then tou must beat trainer tower on 7 island