While sealed in a ball by Bibidi, his master was killed, leaving Majin Buu trapped inside. Later, Bibidi's son Babidi started gathering energy to unleash Majin Buu. After turning Vegeta into a Majin, enough energy was generated by his and Goku's fight that Majin Buu was released. Both Supreme Kai and Gohan (who tried to stop Majin Buu from being released) were no match for him, and were defeated.
Vegeta was unable to stop Buu as well, and in a last-ditch effort killed himself by self-destructing, although Majin Buu regenerated, make Vegeta's sacrifice vain.
For a while, the world lived in terror as Majin Buu decimated the world under Babidi's control. However, after a while, he turned against Babidi and killed him. Majin Buu then went about on his own free will, and later built a house out of people he turned into clay.
Later, when sent to kill him, Mr. Satan befriended Majin Buu, and convinced him to stop killing people. Majin Buu even found a dog and healed it, naming it Bee. Majin Buu's peaceful life was short lived though, as a gunman named Van Zant shoots Bee, enraging Buu, who releases steam. Satan then defeated Zant and his friend that was with him, and Majin Buu was able to heal Bee, who was still alive.Later, Smitty enteres Buu's house and shoots Mr. Satan, enraging Buu once again and starts releasing steam. After healing Mr. Satan, he convinces him to leave as he expels the evil side of him from his body in the form of steam, which forms into Evil Buu. The two fight, and Majin Buu, loosing, fires a Chocolate Beam at Evil Buu in an attempt to defeat him. The plan backfires, however, as Evil Buu blows it back at Majin Buu, turning him into chocolate. Evil Buu eats Majin Buu, and transforms into Super Buu.