The plot of this game is a tad more complicated than those of the previous games.
It all starts with Mario and Luigi going to a very important meeting with all of the rulers of the Mushroom Kingdom. Mario and Luigi were invited because they help protect the Mushroom Kingdom and beyond. While Princess Peach is discussing the new Toad Brigade battle tactic a Luma swoops down and tells Mario and Luigi that they are in great danger. Princess Peach is immediately startled by this news and warns everyone to go back to their posts quickly. However, Princess Rosalina appears on the scene and tells all of them of the Edge of the Universe and the "Big Bad" one who is after the great power. Mario, Luigi and Peach all realize who this "Big Bad" one is and are immediately ready to bring him down. But just as the two brothers get ready to stop Bowser, an airship comes from the sky and lands right outside of the castle. As everyone runs outside Rosalina simply stands there and shakes her head. Bowser Jr. then jumps up from the ship landing on the ground scaring everyone. Next the Koopalings all follow completely scaring everyone even more. Rosalina misunderstood the danger of the situation an flies outside to everyone's defense. Jr. then signals his siblings by pointing at the wands that the Kings had and then at the wand Rosalina was holding. The Koopalings then ambush the Kings and steal their wands and with one giant leap Bowser Jr. goes for Rosalina's wand. However just as he does Rosalina throws her wand up and Bowser Jr. and the Koopalings freeze in place. Everyone else startled from this sight stand motionless. Rosalina then charges her wand until it gleams and sends the Koopa siblings flying into deep space.
The Kings, all troubled, decide to ask Rosalina what that was all about. Rosalina than decided to bring everyone back inside Peach's castle because it was going to be a long story. Rosalina then explains:
The reason why those koopa creatures attacked me was because the "Big Bad" one believes he has found the ultimate source of power. He believes he has found the Galatic Star. The Galactic Star is a large mass of power lurking in the depths of the Galactic Empire, which is a city that hid itself from mankind and any other forms of life Eons ago. Once the power is obtained any form of life that has it will be able to wipe out whole galaxies with the blink of an eye. However, I knowing the cosmos and the universe very well knew that a day like this would come, so I used my power to to put an invisible cosmic shield around the location where the Galactic Star lays. Then I locked it with my wand so no one would ever find it. But Bowser thinks otherwise, and now they will stop at nothing to get my wand and unleash the power.
After hearing this Rosalina wants Mario and Luigi to go stop Bowser. She waves her wand and turns the floating airship into a colorful and jolly Starship. The Mario Starship Version 2. Peach then objects to the brothers going alone and volunteers to go with them. Toad quickly follows behind the Princess wanting to join the journey as well. Rosalina trusts the four with an even greater magnitude than before. She takes the Kings and Queens inside the castlee and puts a protective barrier around the castle, as she would watch over it while the Mario gang is in space. The Starship then takes off into space.
But will they make it in time?
