Beat the elite four, go back to where Misty's city is. There is a road north, go up it. The is water at the end surf it once you get to a cave opening, there will be a cave and you search for Mewtwo...
Mewtwo is in the game. First you have to learn cut. Then go to cerulean city to go to cerlean cave. Last go to the last part of the cave and you will see mewtwo. That is why you have a master ball,...