What do you do in Pokemon fire red after defeating the elite four second time?

Well after you beat it the second cave go to cerulean city go through the bridge in the north go left and down till u can surf. surf down and left when you can and there will be someone blocking it. If ou beat him 1 time it wont work. if second he should let you in. then in some place that is there should be Mewtwo. Well ive only telled you how to get mewtwo. well the answer to your question you unlock mewtwo.