Pokemon: Pikachu, Kore Nan no Kagi?

The Key Pokémon Cleffy appears before Pikachu and friends! When keys that possess mysterious powers are inserted into the keyholes floating in mid-air, the keyholes light up and expand and a magical subspace spreads out before the Pokémon! Cleffy has keys that lead to all kinds of worlds. What kind of world does the key Pikachu receives take the Pokémon to?