You just moved-out of your parents house to start your journey. You then go off into the tall grass to go look for one. You then come across Mewtwo. You have to fight him to try and tire-him-out to catch him. The fight is long and hard, but you end up making him extremely tired and catch him in your Pokeball. When you come Home, you release him from the ball. As he seems quite pissed and yet respectful that you caught him. As he just stands there with his arms crossed. As he avoids eye-contact