Its Princess Peach's birthday but things go horribly wrong. Bill Cipher returns and he's hungry for not just universe but multiverse domination. Thanks to Springtrap, Bill was able to revive. They orchestrate an invasion of the Mushroom Kingdom. Everyone is in fear. Mario and Luigi must save their kingdom from the wrath of Bill. During that time, they meet Dipper and Mabel and later they go into a world where the people there might just help them.