Dialga rules time, and Palkia rules space, they lived in an alternate dimension so they decided to fight within their own reality, which would make it bad for the real world too. Dialga and Palkia appear before Ash and his friends and drag the city into their own world. If they destroy this city, they'll also destroy the whole world.
Source: ANN
Sonans (Musashi)
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Celebi is a mysterious Pokemon, that can travel through time. While escaping from a Pokemon Thief, he transcends through time accidentally bringing a boy named Yukinari with him 40 years into the future. Ash and the gang meet Yukinari and Celebi and heard his story about traveling through time, but even 40 years in the future, there are still some who wants Celebi for themself…
The Pokemon master Ash must rescue his mother from the encasement of a crystal tower.
A strict rule has been imposed in the deep jungle of Okoya Forest where Zarude dwells. One day, Zarude discovers a human baby and names it Koko, deciding to adopt it in violation of the rule. When Koko turns ten he encounters Satoshi and Pikachu which makes him start questioning who he is. But then one day, the sound of unwelcome human footsteps draw closer to the forest.…
Constantly moving throughout the ocean the Temple of the Sea exists invisible to human eyes. It is here that the Sea Crown exists. Phantom the Pirate has captured the egg of a Manaphy, the only Pokémon able to locate the temple. However, his plans are ruined when Jack Walker, a Pokémon Ranger, interferes. Along the way, Ash Ketchum and his friends manage to get caught up in…
Pokémon Trainer Satoshi has a new plan: see the world! But first, he and his partner Pikachu are headed to the opening of the Cerise Laboratory, a research facility dedicated to uncovering the mysteries of Pokémon in every region. Ash meets Gou, another boy with boundless curiosity about Pokémon, and both are overjoyed when Professor Sakuragi asks them to become official…
The XY series is based on the events of the Generation VI games. As with the previous three series, the XY series begins with only Ash Ketchum heading off to a new region with his Pikachu; his previous companions, Iris and Cilan, have left the series. Ash's journey through the Kalos Region begins in Lumiose City, and soon he will be joined by Serena, Clemont, and Bonnie.
Pokémon or Pocket Monsters (often referred to as Pokemon: Gotta Catch Em All to distinguish it from the later series) tells the adventures of Ash Ketchum (Satoshi in the original Japanese version) as he travels through Kanto, the Orange Islands, and Johto to become the greatest Pokémon Master. For the English language release, Pokémon was split into four separate series…
The mini-saga follows Satoshi venturing into an archipelago of islands called the Decolora Islands with Iris and Dent.
Source: Bulbapedia
The Sun and Moon series follows Ash Ketchum on his new journey in Alola with his companion, Pikachu. He and his mother, Deliah, leave for the Alola region on a vacation. However, once seeing the Pokémon School, Ash decides to stay in Alola on a new journey.
From there he adventures across the region with his new RotomDex, as well as his new classmates.