Pokémon Island is a beautiful island. All Pokémon and other friends of Pokémon get in. But NOT the haters, like Bowser. In the past, there were no Pikachus. Samus goes on an research and found a Pikachu-egg. Now, the Pikachus are mutant-rats. Only one Pikachu was normal. It's Mario's task to save him. Actually, Mario and Luigi on Pokémon Island is the first game which Pokémon Island appears in. Recently, a huge flood sank the whole island. Everyone there drowned.
Pikachu now lives with Ash Ketchum and his alter ego Ash Kectchup, however Pinkachu found another more beautiful island and they moved there, unfortunately the mutant Pikachus and their their president found the island and took control of half of it, so Pinkachu has to fight hard and train hard everyday to protect his half and take the other half.