The video starts with Bowser explaining to Bowser Jr that Kamek left after the events of "Dry Bowser's Revenge". Bowser Jr comforts his Dad and heads off to his room. Meanwhile, Koopa travels into a dark room though a portal and hears whispering. Koopa is shocked by seeing Papyrus tied up by Sans. Sans knocks out Koopa since he knows that Koopa is traveling through warp zones. Later on in the forest, Dry Bones feels sorrow for the loss of his father but stops when he notices Koopa tied up. Koopa tries to tell him to run but can't due to his mouth being covered up. Dry Bones runs to Bowser Jr's house and alerts him about the situation. Bowser Jr runs off to find Koopa, much to Dry Bones's dismay. They find that Koopa is missing but they still intend to look for him
Meanwhile, Mario sits on the couch while Luigi criticizes him for being lazy. Mario quickly heads off to his room where he encounters the Mario Amiibo. The Mario Amiibo tells him about a "great evil" who is ready to reveal his sadistic plan. Later in a dark forest, Mario, Luigi, Dry Bones, and Bowser Jr encounter Sans who has tied up Koopa and Papyrus. They demand answers and Sans explains that "they" chose Sans to guard the warp zone to prevent the Shadow Plague from crossing over and eventually, they gave him Papyrus. Sans became bored of the powers he received and guarding the warp zone for eons, he plans to gain more. Sans realizes that he could absorb souls to become more powerful and hopes to destroy dimensions connected to the warp zone. Sans believes since he was chosen to guard the warp zone, he was chosen to become the most powerful being in the universe. After that, four souls appear in the sky as Sans absorbs them.
Sans transforms into a god-like being that could control time and contains thousands of dimensions. The Mario and Link amiibo arrive and explain that they can't do anything to stop it since it's too powerful. Mario attempts to attack but he gets zapped as a result. Before believing there is nothing they can do, Nabbit arrives and wants them to distract Sans while he runs towards him. As Nabbit runs, Mario and Luigi zap their fire and zen balls and Nabbit jumps into Sans. Nabbit enters the core of the God-like Sans which Sans says that there is nothing left to defeat him. Nabbit says that he has been through pain and embarrassment himself, hopping to make Sans feel it too. Nabbit pulls out a gun and shoots the core which explodes, killing Sans and Nabbit.
After that, Mario, Luigi, Bowser Jr, Koopa, Dry Bones, and Papyrus stare at the grave stone of Nabbit which Papyrus appears to walk away. The next day, Mario and Luigi are sitting on a couch where Luigi wants to know what they'll do today.