There is a small and rather unsettling Easter Egg that can be found in the basement of Pokémon Mansion in Cinnabar in Let’s Go, Pikachu! and / or Eevee!, which relates to the backstory of the mansion and the legendary Pokémon elaborated on within.
You can find this piece of lore building only by approaching the large, unexplained machine pictured below, found in the mansion’s basement, with either Mew or Mewtwo as your active traveling companion outside its Poké Ball:
Mew and Mewtwo are the only two Pokémon in the game that will react in any way to this machine’s presence. Both will give a signal of surprise and approach the grim-looking machine, in turn prompting the player to speak to them for the accompanying bits of dialogue at the top. It’s a very small and easy to miss sliver of continuity but to me it speaks volumes about what must have happened in that mansion before it burned.
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