Mario and Luigi were fighting Antasma in the Dream World, something happened, the stress of the magic was too much for Luigi's mind, injuring Luigi in the real world, and now the Dream World along with the Luigi clones are rotting along with Luigi's corpse in the real world

But Luigi, with little life left refused to die, essentially becoming a living corpse but does his best to help Mario escape from the dying dream world while the Luigi clones become violent due to what happened to Luigi