Big Nyarth Day (ビック・ニャース・デイ) features animation from Pokémon 3D Adventure 2 Pikachu's Ocean Adventure (ポケモン3Dアドベンチャー2 ピカチュウの海底大冒険). The story for that the short involves Pikachu and the other Pokémon go to an island for some fun until a Perap discovers a treasure map and the Pokémon decide to follow it. The ending song lyrics do not relate to the actual short but use the animation to create a new story that revolves around Nyarth's day at the tropical island.
The ending starts off with a wave rolling up to the shoreline as it transitions to a rock with a bunch of water Pokémon resting on it as a Wingull and Pelipper fly over in the sky. Satoshi, Haruka and Takeshi's Pokémon as well as Rocket Gangs Pokémon are playing with a ball that looks like a watermelon on the beach as Nyarth sits in a beach chair with tropical fruits beside him. The beach ball is hit towards Nyarth and the the fruit on the table goes flying.
The next scene returns to Nyarth sitting on his beach chair with the tropical fruits beside him as he relaxes in the sun. The next scene features Nyarth and Pikachu on top of a Wailord with the map as it spurts out water as Latios and Latias hover around.
Latios and Nyarth are seen underwater with Pikachu on Latias win a part CGI scene of the ocean floor. The come up to a statue of Solrock and then swim away.
Pikachu and Nyarth are seen inside a sunken ship as a Huntail swims by and scares them. A Sharpedo then chases them from behind and they quickly start swimming away.
Nyarth is once again riding on Latios and Pikachu on Latias as they encounter a Manaphy. They follow Manaphy as they pass by a Lugia but on the way, Nyarth cannot hold his breath any longer and starts to struggle for air.
All the main character Pokémon are seen playing with a bubble underwater near the sunken ship in the next scene. Pikachu and Nyarth kick the ball and the final scene shows Pikachu and Nyarth waving as Nyarth says 'Thank You!'.