Instructions
Obtain all eight badges and defeat the Elite Four. Capture over 60 Pokemon and speak with Professor Oak in Pallet Town to obtain the National Pokedex.
Retrieve the Ruby from Mt. Ember and the Sapphire from the Rocket Warehouse on Five Island. Fly to One Island and give the Ruby and Sapphire to Celio to fix the Network Machine.
Bring Pokemon that know the moves Surf and Strength and go to Cerulean City. Cross the Nugget Bridge to Route 24 and Surf into the Cerulean Cave.
Surf into the river. Follow the river path and drop down to the land at the south end. Ascend the stairs.
Follow the path and ignore the first ladder. Descend the stairs and climb the next ladder. Head south and turn right, using Strength when necessary.
Ignore the first intersection and continue along the path until you reach a ladder. Climb the next ladder and descend the stairs.
Follow the path to the river and Surf to the small island where Mewtwo awaits. Save the game and confront Mewtwo.
Lower Mewtwo's HP into the red and afflict it with Paralysis, Sleep, or Freeze. Throw Ultra Balls or use the Master Ball to capture Mewtwo.