After the defeat of Bowser and the restoration of peace in the Mushroom Kingdom, Mario and Luigi return to their normal lives as humble plumbers in Brooklyn. However, their tranquility is short-lived. A mysterious cosmic rift opens in the skies above the kingdom, spewing out stars of corrupted energy and summoning strange enemies from different universes. At the heart of the disturbance is a fractured galaxy ruled by a powerful sorceress—Cursa, a new villain obsessed with absorbing star power and rewriting the rules of every world she touches. Princess Peach summons Mario and his friends, sensing a far greater danger than Bowser ever posed.
The journey takes them beyond familiar lands into new dimensions. Riding on the back of a newly discovered Yoshi, Mario and Luigi venture into the Sky Realms, a cluster of floating islands ruled by Rosalina and the Lumas. Rosalina reveals that the cosmos is unraveling due to Cursa's tampering with the Grand Comet, the source of all balance in the universe. With the comet weakening, the dimensions begin to merge, causing enemies from Donkey Kong’s Jungle, Luigi’s Mansion, and even Wario’s twisted world to cross into the Mushroom Kingdom. The only hope lies in assembling the Seven Galaxy Shards, ancient pieces of the comet hidden across various chaotic realms.
Meanwhile, Bowser, now imprisoned in a magical snow globe, senses the chaos and plots his escape with the help of Kamek and an accidental assist from Wario and Waluigi, who want the galaxy's power for themselves. Forced into an uneasy alliance, Mario, Donkey Kong, Peach, and Luigi must team up with unlikely allies like Rosalina, Daisy, and a sassy robotic Toad named Mecha-T. As they journey from the fiery Volcano Gardens to the icy halls of Crystal Cavern, each character confronts personal fears: Luigi faces a haunted realm of illusions, Peach must sacrifice her crown for a chance to reset time, and Mario is tested by visions of losing his brother forever.
The final battle erupts on a shattered moon orbiting the comet’s core. Cursa transforms into a monstrous version of herself, fueled by the stolen shards. Just as all seems lost, Yoshi channels the energy of the Grand Comet, unleashing a cosmic stampede of multiversal creatures to aid our heroes. With friendship, clever teamwork, and a little help from a power-up no one has seen before—the Infinity Star—Mario and Luigi combine their powers to defeat Cursa and restore the galaxy. As the worlds are separated once again, Mario promises Rosalina they will meet again. Back in Brooklyn, the brothers reopen their shop, now with a glowing star over the door… just in case the universe needs saving one more time.
