How to Find and Catch Mewtwo in Pokémon HeartGold and SoulSilver

Cerulean Cave

Defeat the Elite 4 and the champion, Lance.

Get to Cerulean City and find Cerulean Cave. Remember the location of it.

Defeat the rest of the gyms.

Go back to the cave and find your way through it.

You'll see Mewtwo. Go up and talk to it, then get ready to catch it.

Catching Mewtwo

Stock up on Ultra Balls, Dusk Balls, and Timer Balls.

Get a Psychic Pokémon and a Steel Pokémon.

Battle Mewtwo until it's at less than 50 percent of its health.

Throw your Ultra Balls and Dusk Bulls until it's caught. If the battle goes on for over 30 turns, throw your Timer Balls as well.