Things You'll Need
Instructions
Enter Cerulean Cave, located just outside Cerulean City. You need to have beaten the Elite Four, have acquired the National Dex, and have given Celio the Ruby and Sapphire to be allowed to enter the cave.
Enter the water using the surf skill on your Pokemon.
Surf to the far west side of the cave. Ignore the water exits you see.
Exit the water on the west side of the cave to the south.
Walk up the stairs to the east platform.
Walk past the first ladder on the east platform.
Climb up the second ladder located on its one separate platform. This will take you to the second floor of Cerulean Cave.
Walk to the ladder on the west side of the cave. There are numerous ladders visible on the second floor, but the path will only allow you to go to the correct ladder. All of the other ladders are on blocked pathways.
Climb down the ladder to return to the first floor.
Climb down the ladder just to the south of your current location to go to the basement.
Walk through the cave until you reach the stairs leading down to the water. There is only one path through the basement.
Enter the water using the surf skill.
Surf to the platform to your west.
Talk to Mewtwo to start the battle with him.
Use the Master Ball on Mewtwo. You do not need to damage him first to capture him. The Master Ball will always automatically catch the Pokemon it used on.