If you live in North America and happen to own a copy of Let's Go, you'll be able to download the mighty Mewtwo at your local Best Buy store from 4th - 14th November with the assistance of the Pokémon Pass app for your iPhone or Android device.
From here, you boot up the game, select communicate from the main menu > mystery gifts > get with code/password and then select 'Yes' to save your game and connect to the internet. You then enter your code and watch as your gift arrives in the game.
Are you still playing Let's Go on Switch? Are you in need of an extra Mewtwo? Tell us below.