basically, a legend had it of a second world, one that one could not enter, and mario made it his mission to discover this world, so by following the legends' directions, he made it to a cave, where he would find a pipe, a pipe that would take him to this new world, one simply referred to as the minus world, but the world's energy wasn't like that of which he had seen before, and it had began to corrupt and take over Mario as it did with anything that came into contact with it, Turning him into a Monster hellbent on bringing The Minus Plains into The Regular world. with luigi being the one to stop Mario from destroying their world