They'll start with the old Game Boy titles, and then make their way up into the DS era, campaigning the whole time to try and raise money for charity.
The marathon will begin at noon on Saturday, and each of the three streamed screens will be running a different version of Pokemon, so you'll get to see for sure just how different the game is if you go with Bulbasaur, Charmander, or Squirtle for your first pet. The infamous Pokémon bootlegs will also be making an appearance. All the money will be going to Free the Children, so it's for a great cause.
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