Sayonara, Pokémon Master! (さよなら ポケモンマスター Sayonara, Pokemon Master!) is a fan-made series of the Pokémon anime, it's chronologically set shortly after the events of finale of the Pokémon: Diamond and Pearl season, effectively retconning the events of all subsequent seasons afterwards.
Like the canonical anime, the great majority of the story centers around Ash Ketchum, a Pokémon Trainer from Pallet Town, and his partner Pikachu traveling across the world aiming to become the worlds greatest Pokémon Master. After his defeat in the Sinnoh League, he takes a different approach after being accepted into a group known as the League of Masters, an organization dedicated to cultivating the next generation of Pokémon Masters.
Sayonara, Pokémon Master!
After soon parting ways with Dawn and Brock in the Sinnoh region, ten-year old Ash Ketchum and his Pikachu once again return back to Pallet Town. While he still aims to become a Pokémon Master, he lacks a concise idea on what he should be doing next in his journey or where he should go.
Those thoughts are abruptly ended after meeting an old friend of Professor Oak's, Alder the Champion of a region known as Unova. Ash is amazed by Alder's Bouffalant and challenges him to a battle against his Pikachu, and loses. Regardless, Ash is still informed about a region in between Kanto and Johto known as the Sinjoh Archipelago run by a group known as the League of Masters, and that whoever challenges their league will become an official member of their group.
Ash immediately expresses excitement in the League of Masters and heads off to the Sinjoh Archipelago with Pikachu, leaving his Toreterra, Gliscor, and Buizel at Professor Oak's Lab in order to capture some new Pokémon.
Meanwhile, the Team Rocket Trio finally return back to Team Rocket HQ after their "efforts" in defeating Team Galactic and Hunter J, and are promoted by Giovanni, who gives them an important mission of traveling to the Sinjoh Archipelago to take down the League of Masters at all costs. Giovanni reveals that his true agenda is to find and capture Arceus, additionally harnessing its powers and becoming a dictator of the world.
Giovanni specifically commissioned Team Rocket scientist, Dr. Zager, to create a mind control device that will brainwash any Pokémon of his choosing and do his bidding, which he plans to use on Arceus after finding its location.
Upon traveling to the Sinjoh Archipelago, Ash learns about the league competition and its rules, whoever travels the multiple islands facing off against the five Pokémon Masters of those respective islands and gains their Battle Print, they will gain the opportunity to face the leader of the League of Masters and be admitted into their ranks.