A long time ago, in a galaxy really far away, existed this really creepy planet. This was the planet of the Shroobs.
The Shroobs then came to conquer the Mushroom Kingdom, first by turning the Toads into fuel for their flying saucers, and then by Shroobifying the Mushroom Kingdom. However, little did anyone know that the Shroobs were bent on galactic domination. This would probably be a good thing, though, considering the Shroobs were deliberately destroying the Mushroom Kingdom.
Unfortunately, the Shroobs nearly went extinct thanks to the young Mario and Luigi and their time-traveling adult counterparts.