"The Mirror Border" originally submited during the first SMW Central HackJam.
This is a story-heavy hack. The main theme is duality, specifically the duality between light and darkness. The world map will be divided in two: Light and Dark with the Mirror Border separating the two. Each world is an inverted reflection of the other, for instance water in one world is lava in the other. The Mirror Border cannot be crossed, so in order to explore the dark world, you will need to take control of Dark Mario.
"A mysterious barrier appeared close to one of the towns in the Mushroom Kingdom. There is no apparent way to break or cross the barrier, but it's possible to see a dark reflection of our world at the other side.
A group of researchers is currently investigating this barrier and found it to be a potential threat. The light and dark worlds once separate are colliding. The border between the two words is weakening, and if it were to break, the worlds would crash into each other and nothing would remain.
Therefore, the researchers have tasked Mario to find a way into the border and fix this impeding threat. But Mario can't do this alone. In order to ender the barrier, Mario must gather items from both words, and like every other inhabitant from the light world, he is unable to cross to the dark side. In order to save both words, Mario will need the help of his twin from the dark word."
NOTE: Due to how the game handles 2 Player mode, it's impossible to save the game (which is the reason the hack length is 0 exits). The game actually has 14 exits. The hack uses SA-1 so use savestates at your own risk. The hack is perfectly beatable in under 2 hours without saving (via savestates or otherwise).