This episode is less of an episode and more of an introduction/celebration of the world of Pokémon.
Red catches Pikachu in Viridian Forest! How lucky, considering their rarity.
Basically this short is just Red and that Pikachu battling Pokémon across different regions, going from Kanto to Kalos. They battle a Caterpie in Viridian Forest, some Wooper outside Ecruteak, a Vigoroth by the Weather Institute, Probopass at Snowpoint Temple, Volcarona in Relic Castle, and Noivern at I-forget-the-name-but-it's-the-place-you-catch-Zygarde (edit: Terminous Cave! Just remembered). Each time, a cameo legendary shows up.
Yeah, this one's mainly a nostalgic teaser of what's to come. I, for one, was incredibly happy to see the Weather Institute and Relic Castle, since they're some of my favorite game locations. I also have a ton more respect for Probopass now.
All in all I give it a "Wow, Pikachu sounds weird when it's saying something other than its name"/10.