To catch Mewtwo, first you must beat the elite four and then get the sapphire and the ruby to give to Celio. You get the ruby from a secret entrance into Mount Ember on 1 island (which will be being guarded by two Rocket Grunts). Once you beat them (which won't be hard)they will tell you one of the two codes to get into the Rocket warehouse on another island. Then go into the cave and move through it. There won't be any battles and it doesn't take long. At the end there is the ruby, waiting for you to pick it up. Take it and leave. Give it to Celio in the 1 island Pokemon Center and go to the island with the brail cave. In the Pokemon Leaf Green or Fire Red box, there is a brail booklet. You can use it to crack the code on the Brail cave entrance door which will read "use cut". Use cut and enter. Inside there will be a hole. Drop into it and you will land in another room with four holes. At the back of this room there will be another brail stone. Use the brail booklet and read it. It will say up, down, left or right. If it says right, drop into the hole on the right, if it says up, drop into the hole at the top of the room etc. Underneath this room there are three more rooms that will have brail stones reading different directions. Eventually you will land in a room with the sapphire on a table. If you attempt to take it, a Rocket guy will charge into the room and steal the sapphire, claiming it as his own but telling you the second code for the Rocket warehouse. To get it off him, you must go to the island with the Rocket warehouse and beat everyone in there, including Giovani's second in command. Before battling you he will go on about Giovani leaving. Beat him and go into the next room. The guy who took the sapphire will be sitting at a table. Talk to him and he will give you it to you. In turn, give it to a delighted Celio and then go back to Kanto. Fly to Cerulean city and enter Cerulean Cave. Go through the maze and find Mewtwo. He is extremely difficult to catch, but it's worth the trouble! Of course, it won't be hard to catch him with the Master Ball if you saved it. If not keep trying and you might eventually catch him. He knows Psychic, Recover, Barrier and Swift.