The story is the driving force behind Mushroom Kingdom Fusion, explaining why all the characters are in their predicament and how it all fits together. Unlike most fangames, this game boasts a very dark and complex storyline, featuring a plentiful number of classic video game characters as well as introducing some original ones.
The Super Mario Brothers arrive at Princess Toadstool's castle to find that Bowser has kidnapped the princess yet again! The Bros. will have to brush up on their skills and set out on yet another journey to rescue Princess Peach from the villainous Bowser and his army of minions! It's a classic-style Mario Bros. adventure once again!!
....Or is it?
The adventure turns out to be anything other than ordinary as they discover a hole in the fabric of time and space that has begun fusing together their beautiful Mushroom Kingdom with strange other worlds from throughout the multiverse! The Bros. are also approached by a mysterious figure, who wishes to recruit the duo and several other heroes into a group known as the Coalition, whose goal is to stop all of reality from being fused together and assimilated by the eldritch horror known only as Legion.
The Coalition members aren't the only ones banding together, unfortunately, as a number of prominent super-villains have also banded together under Legion's banner with the intent of furthering the Fusion process! Can these ten brave heroes stop the countless forces of Legion and the Villain Alliance to return all the worlds of the multiverse to their original states??
The plot of Mushroom Kingdom Fusion is still under development, and will be kept strictly under wraps until the game is completed.