Same as SMG2.
Mario and Luigi are on their way to visit Peach. When they arrive in Toad Town, they discover a humongous comet is approaching. On top of the comet is Giga Bowser. Giga Bowser grabs Peach as the Toads flee. Mario and Luigi begin to run to save Peach, but a small meteor lands on them, crushing them. Kamek then appears and blasts them out into space...
Mario awakens on a small planetoid, where Destello is. Destello is happy that he's awake. Luigi wakes up next to Mario and Destello takes them to the SS Rosetta, where he introduces them to Rosetta, Twinkle and a few more Lumas. Once arriving in the engine room, where the Apricot Luma is. Hazel is chasing Luma around, trying to destroy the engine. After Mario defeats Hazel, Luma joins Mario with the same powers as last time. Hazel decides that she's tired of working for Bowser- low pay, even lower food rationing, ect., and joins Mario's team. Mario, Luigi and Hazel are all greeted by Rosetta, making sure everything's ok (though she lies that she just wanted to see the excitement). Destello and Luma tell the Mario Bros. that when Bowser's comet flew by, it knocked all of their Power Stars away, keeping them from going anywhere. However, Twinkle reveals that he believes a few Power Stars are in the closest galaxy - the Mountain Climb galaxy. Mario, Luigi and Hazel all agree to explore the Mountain Climb Galaxy, but Rosetta begs to be brought along. Twinkle and Destello are reluctant, but finally let the little girl go with the three others. The four heroes head to Mountain Climb.
|Mountain Climb Galaxy|
|Classic World Galaxy|
|Weapon World Galaxy|
|Larry's Cosmic World|
|Desert Dessert Galaxy|
|Tiny Terrace Galaxy|
|Dental Detatchment Galaxy|
|Morton's Mega-Destructor||Morton's Megaleg Mahem|
Using character stops, you can swap between characters. Only one character will be at each stop, and that character will be required for the next part of of the mission.
- Mario - Main character of the game. Only person who can Star Spin.
- Luigi - Mario's brother. He has higher jumps then anyone else.
- Hazel - The rebel Goomba. She's the fastest character.
- Rosetta - The young star princess. She can float across small gaps.
- Peach - After Rosetta's kidnapped, Peach takes over. She can float across small gaps.
- Bowser - The main villain. He kidnaps Peach, and later Rosetta.
- Bowser Jr. - Bowser's son. He's the boss of World 7.
- The Koopalings - Bowser's children. They're the bosses of different worlds.
- Ludwig - Boss of World 6.
- Lemmy - Co-Boss of World 5.
- Iggy - Co-Boss of World 5.
- Roy - Boss of World 4.
- Wendy - Boss of World 3.
- Morton - Boss of World 2. (Morton's Mega-Destructor)
- Larry - Boss of World 1. (Larry's Cosmic World)
- Koopa Brigade - They replace the Toad Brigade.
- Squeek - A Penguin. He's like Koopa T. Quick, II Piantissimo, Spooky Speedster and Cosmic Mario and races you when there's a Dash Comet (grey).
- Destello - The captain of the SS Rosetta. He gives you tips and acts like Rosalina/Lubba.
- Twinkle - Rosetta's assistant. He acts kinda like Polari.
- Luma - The main Luma. He joins Mario after Mario defeats Hazel, seeing him worthy. He allows him to star spin like in the first two games, but doesn't help Luigi, Hazel, Rosetta or Peach.
- Cosmic Clone - Appears when a Cosmic Comet appears
- Reznor - Appear in Dental Detatchment Galaxy
- Bob-Omb - Appear in Weapon World Galaxy
- Bullet Bill - Appear in Weapon World Galaxy
- Gargantua Koopa - Appear in Tiny Terrace Galaxy
- Megaleg - The boss of Morton's Mega-Destructor along with Morton.