500 years have passed, and all of the original Mario and Sonic characters we’ve known and loved have died off. In that time, the portal access between Möbius and the Mushroom Kingdom has been closed off for unknown reasons... but now it has opened up once more after 200 years of not having access
In the time that has passed, since All the humans in the world have died off, the Mushroom Kingdom is now ruled by Toads, and have made peace with the next door Koopa Kingdom, and so everything is peaceful, or at least it appears that way.
We follow Violet and Shellock, a Toad with a purple head and a koopa respectively, both the children of the current ruling families, who have decided to step into the neighboring universe for the very first time...
(And we go from there)