Original Lore:

Long ago in the Galar region, there was a terrible disaster called the darkest day. During the darkest day, all pokemon became huge and powerful, AKA Dynamaxing. People didn’t know what to do until one day 2 heroes came one held a sword another held a shield. And they stopped the darkest day with the power of the legendary pokemon Zacian and Zamazenta.

DLC story:

Right now we only know the story for the isle of armor. So pretty much you are going to be trained by the former champion and mentor of Leon, his name is Mustard. We will be training in dojos alongside the pokemon Kubfu. Kubfu, evolves into the legendary pokemon Urshifu. Urshifu has 2 forms/styles that he evolves into single-strike style a fighting and dark type and rapid strike style a water type. On Pokémon Direct I noticed that there were 2 dojos a red and blue. I believe that if you train Kubfu in the red dojo it will evolve into a single-strike style and if you train in the blue dojo it will evolve into a rapid strike style. In part two: the isle of armor you will find more legendaries including the new legendary Calyrex. There will also be new forms of the three legendary birds Moltres, Zapdos, and Articuno.