The game begins with a cutscene. This cutscene takes place in an unnamed city where four Pokémon are playing: a Charmander, a Chikorita, a Piplup and a Pikachu, which will later become the character you control. They appear to be playing tag when a Mew suddenly appears. The Mew takes them to a mysterious hole and the Pokémon are accidentally pushed into it. They all launch into a free fall through a black area with multiple colors shining around it before landing in a new area known as the PokéPark. At this point, Mew finally begins to explain things to you.
Basically, Mew brought you to the PokéPark to gather sixteen Prism Pieces. These pieces belong in the Sky Haven, but have fallen off of it due to the lack of friendship between Pokémon recently. Along this journey, you should relocate and save your friends, who have been lost after falling through the hole. With this introduction, you finally get to begin your game.