Mewtwo is a legendary pokemon that you have to catch in cerulean cave. to get to cerulean cave, you must defeat the elite four, get the ruby in mt ember, and get the sapphire in the dotted hole. when you get the ruby and sapphire and bring it to celio in one island in the pokemon center, the cerulean cave will be open. but, be carefull because 39 to 68 pokemon like wobbefet, magneton, parasect, etc.
from
the pokemon master