Bowser, tired of being defeated, decides to find a way to finally defeat the Mario brothers once and for all. Knowing that the most powerful energy source in the universe is the sun, Bowser orders the Koopa Troop to begin construction of a machine that can absorb the energy of the sun. This machine is also enhanced with magic by the Magikoopa's in the Troop. After turning on the machine, however, it absorbs the entire sun and overloads, exploding and sending the Koopa Troop flying.
Meanwhile, Mario is sleeping when Toad wakes him up, and tells him that the sun has disappeared. Mario rushes outside, and sees that the sky is dark. He decides to go to the castle to check on the princess, but along the way he encounters a Power Star; after collecting it, it flies into the sky and turns into a small light. Mario continues along to the castle. When he arrives, Rosalina is waiting for him. She tells him that the Koopa Troop used a machine to absorb the sun, but it failed, and ended up seperating the sun into a number of Power Stars which were scattered across the kingdom. Luckily, Rosalina got some Luma's to become Launch Stars that lead to where the Stars are located. Mario sets off to collect the Power Stars and recreate the sun!
As in Super Mario Galaxy 2, Yoshis appear in the game and can be ridden on by Mario or Luigi. However, unlike Super Mario Galaxy 2 where Yoshi could gain special powers by eating certain fruits, in Super Mario Radiance there are multiple different colours of Yoshi, who all have a different ability.
When an egg has a certain colour of spots, a Yoshi of that colour will hatch from it. To change the colour of the Yoshi you are riding on, you can find buckets of coloured paint throughout the levels. Grabbing this paint will change the colour of your Yoshi to that colour, and give them all the abilities that come with that colour as well. However, when the Yoshi touches water any paint it has will wash off and it will return to it's original colour.
- Luigi: After collecting a Green Power Star
- Wario: After collecting all of the Power Stars and Green Power Stars.
- Yoshis: Ridable
- Green Yoshi: No special abilities.
- Red Yoshi: Runs extremely fast; can run on the surface of water and up steep slopes.
- Pink Yoshi: Bounces around.
- Yellow Yoshi: Reveals secret paths when it goes near them.
- Light Blue Yoshi: Is a lot lighter; can jump higher, and get carried around by strong winds.
- Blue Yoshi: Swims extremely fast.
- Orange Yoshi: Spits fireballs instead of throwing eggs
- Black Yoshi: Can eat enemies that usually damage other Yoshis (such as spiky or flaming enemies).
- White Yoshi: Thrown eggs will explode.
- Princess Peach: The princess of the Mushroom Kingdom, she helps Mario and Luigi out by telling them about hidden stars they haven't found yet.
- Toadsworth: Princess Peach's advisor, he gives Mario and Luigi tutorials on various things in the game, and also teaches them new skills.
- Rosalina: A mysterious woman who raises the Luma's, she gets her Luma's to create new Launch Stars to let Mario and Luigi reach new places.
- Polari: Rosalina's assistant, he will show Mario and Luigi which Power Stars they have collected from which levels.
- Toad Brigade: A group of Toads who assist Mario and Luigi in collecting the Power Stars.
- Red Toad: The leader, he is a bit of a coward. However, his helmet has a light on it that can supply light in dark places.
- Blue Toad: A smart Toad that gives Mario and Luigi useful hints.
- Mailtoad: This Purple Toad will give Mario and Luigi letters, and is good at finding underwater secrets.
- Banktoad: The Green Toad will store items for Mario and Luigi, and will also find underground secrets.
- Yellow Toad: The Yellow Toad is sleeping all the time, so he's not really useful for anything.
- Goombuddies: Goomba's who help Mario out by activating cannons.
- Hello World