From the Super Mario Wiki
Misaki is a martial arts master who resides in the Oriental city of Sayonara. Misaki is presumably well-known throughout the Mushroom World, as Mario, Luigi, Princess Toadstool and Toad made a return trip to Sayonara to purposely seek him out. Misaki is shown to run a dojo, where he trains himself and any students wanting to learn from him.
When Mario, Luigi, Princess Toadstool and Toad traveled to Sayonara to seek out Misaki, they were ambushed by Karate Koopa and his Ninji minions, who proceeded to tie up Princess Toadstool and Toad. Fortunately, Misaki found Mario and Luigi, and was able to rescue them from the Ninjis. Mario asked Misaki if he could teach them fight like him, and while Misaki pointed out that no one else could rival his skills, he agreed to help them regardless.
Misaki attempted to teach Mario some basic karate moves, but Mario kept getting his head handed to him. Realizing that Mario and Luigi made better plumbers than fighters, he resorted to teaching them Plumb Fu, a form of martial arts used exclusively by plumbers. Mario and Luigi picked up Plumb Fu almost effortlessly, and together with Misaki, set out to rescue Princess Toadstool and Toad.
Before they could reach the pagoda where Karate Koopa had taken up residence, Misaki, Mario and Luigi were captured in a net, and soon found themselves chained to a boulder atop a catapult. Karate Koopa launched the boulder into the ocean, where it sank to the bottom. When Misaki was unable to break through the karate-proof chain, Mario used a hacksaw to cut it. Misaki then fought off a school of hungry Trouters with Mario and Luigi, before escaping from the ocean through a drainpipe.
Climbing out of the other end of the drainpipe, Misaki and the Mario Bros. emerged in front of Karate Koopa's pagoda. Upon entering, they were ambushed by a horde of Ninjis, which Misaki proceeded to fight off with karate, while Mario and Luigi resorted to using Plumb Fu. After hearing Princess Toadstool cry for help from another room, Misaki ordered Mario to go and rescue her, while he and Luigi continued to fight the Ninjis. After they finished, Misaki and Luigi came to Mario's side, only to discover that Mario already took out Karate Koopa on his own.
Back at his dojo, Misaki treated Mario's group to a meal, and presented Mario and Luigi with black belts. Quite humorously, due to the fact that Mario had put on extra weight from gorging on Misaki's Japanese food, he was unable to properly tie up his black belt, and requested for Misaki to give him another one, if only to extend the length of the first one.