From Bulbapedia, the community-driven Pokémon encyclopedia.
The game starts with Pikachu, Charmander, Chikorita, and Piplup playing and running around. When they go into a deeper spot, the area appears to be glowing white. When they reach close to the end, they meet a strange Pokémon called Mew. Mew gets happy at the sight of Pokémon playing and the four of you get happy too, happy because they met such a friendly Pokémon.
Suddenly, Mew forms a hole into the white, glowing ground, which makes Mew even happier. Pikachu, skeptical, looks inside, however there appears to be nothing but blackness. Suddenly Piplup accidentally bumps into you, which causes you to fall down the hole, absolutely terrified.
Nothing but the strong friendship could have made Charmander, Chikorita, and Piplup go after, you concerned about your well--being.
Encounter with Mew
Everything is white and gold, sparkling beautifully. You appear to be in a dream. You look around, wondering where you are, when suddenly Mew appears! Mew, as happy as he was earlier, seems to be quite upset now. He explains that the PokéPark, a place for Pokémon to have fun, is in big trouble now that everyone is acting odd and cruel lately, doing things they would have never done in their earlier state. He must be getting closer to what he really wants to tell us...Mew asks you a huge favor. According to Mew, the Sky Prism was broken, which is why unhappiness is looming over the PokéPark. So he asks if you can go collect Sky Prism Pieces, as he saw you having fun with your friends earlier. And then the dream ends, and you continue to fall...
|| Skill game
|| Along the path to the PokéPark. (only once)
|| Has the player chase him while teaching them about Skill games.
When you (finally) wake up, your eyesight is slightly blurry. You look blearily everywhere, wondering where you are. Then, suddenly, your eyes meet a Chatot, who is looking at you and quite concerned. Apparently, Chatot has never seen you around before, and after you introduce yourself, you ask where your friends went. Unfortunately, Chatot hasn't seen Charmander, Chikorita, or Piplup anywhere.
He asks you if you were heading to the PokéPark, and you suddenly remember about Mew, saying how the PokéPark was in danger and that Mew desperately needed help on getting the Sky Prism Pieces back, so you nod. Chatot then says he needs to test you if you can qualify for the PokéPark, so he flies off while telling you to follow him.
To walk, hold the Wii Remote horizontally (this applies for all other controls) and press the "up" button on the Control Pad.
When you make it there, you will see a large box in front of you, which Chatot says is the first part of the test. He tells you to dash into the box, which you will collect a berry.
To Dash, to same controls as walking well pressing 1. To collect the berry once the box has smashed, walk through it.
Chatot is very impressed by this and flies away, telling you there is more to the test. Curious, you go and follow him.
When you arrive, there is a very large log in the way. Chatot says that this is the second hurdle of your test (literally)--you can run, but can you jump? He then flies over the log, leaving you alone on this side of the log as he waits for you on the other side.
To jump, press 2 (while not facing somebody). To short jump, press 2 very quickly. For long jump, press 2 once the regular jump has reached the highest point.
After completing this, Chatot is now very impressed, although there is another thing he needs to check...
This is the last part of the test--Skill Games. Chatot explains to you that Skill Games are a way for Pokémon to make friends, as well as get some berries. He also says that if you make a friend, it gets added into your PokéPark Pad, which is what he had given you just now.
Chatot says the most popular Skill Game is called Chase. In Chase, you have to either run away or catch the opposing Pokémon--in this case, you're always the catcher. He says that he's going to be the one playing chase with you, and if you beat him, he will be added to your PokéPark Pad and give you some berries.
As the first Skill Game you try, it isn't too hard (however, in future chases it will get increasingly harder). There is a time limit with no certain time; there is no set time limit as it all depends on who you are facing. Slower Pokémon get a longer time limit than others, while the faster ones get shorter ones. This is to make it more challenging. Dash and knock into your opponent.
Once you defeat Chatot, he marvels over how he still feels like he is being knocked out of the air, and then remembers that you're with him. Apparently he wants you to open the PokéPark Pad, since his name is in it. When you open it, you'll notice that you can also save your game data there and check your stats, as well as see the items you unlocked, via password.
After closing it, Chatot gives you some berries and explains more about the PokéPark Pad. If you only did the required Skill Game once, they are only your friend, with a regular-colored PokéPark Pad beside their photo. However, if you do the requires Skill Game twice, they are your best friend, and have a rainbow-colored PokéPark Pad beside their photo!
Chatot then leaves you alone, saying he'll meet you at the PokéPark.
As soon as Chatot leaves you, a Buneary comes to talk to you. She asks if you're going to the PokéPark too, and gets extremely excited. She then leaves you alone, like Chatot, getting excited at the prospect of seeing you at the PokéPark.
The Speech Bubble
When you arrive there, Buneary seems to have an enormous Speech Bubble above her head. This is used for Pokémon that need to tell you something. Usually, if there is a Pokémon with this on their heads, when you approach them it automatically talks for you, but for a lesson right now, press 2 when a "talk" bubble appears over Buneary.
Apparently, Buneary is upset and annoyed since she can't go to the PokéPark since a sleeping Snorlax is blocking the entrance. She asks you if you can use that "Electric shocking move" to wake Snorlax up.
The "Electric shocking move" turns out to be Thunderbolt, according to the game's dialogue. To use it, hold A to charge up and release it when you're ready. For now, Thunderbolt is relatively weak.
Thunderbolt effectively woke up Snorlax, now wide awake. He, like all people when they just wake up, moves a bit slowly and pleads for you not to attack him again. He gets up with a thud, and he goes to sleep in another place, and as he falls down, it makes a loud thump, and a heavy one, since it was strong enough to make you and Buneary to jump quite high. Buneary gets very happy and enters the PokéPark, leaving you alone.
Well, you won't be alone for long, as you're going to follow her and enter the PokéPark!