The tournament goes well, with the final round of the child’s division being Daika versus Trunks. Trunks ends up beating her and wins the champion’s title. In the adult’s bracket, the final match is between 18 and Mr. Satan. Mr. Satan pays off 18 to lose the fight, which lets Mr. Satan remain the champion.
As the tournament wraps up, a new face appears. He introduces himself as the Supreme Kai, and he says he needs their help to defeat a new threat. The dark wizard, Babadi, is on Earth trying to revive Majin Buu. They head off to Babadi’s base.
Babadi meets them with his servant, Dabura. Vegeta dispatches the Demon King with some difficulty, but this also results in Majin Buu being awakened. Babadi then takes over Vegeta’s mind, turning him into Majin Vegeta. Majin Vegeta then challenges Goku, and they fight. During this, Raditz and Gohan go to try and put a stop to Buu. Buu easily defeats even their SSJ2 forms. After fighting almost all of the other Z-Fighters and easily beating them, the recovered Vegeta decides to join the fight, along with Goku. They both power up to SSJ2 and begin fighting Buu. Their combined power manages to overtake Buu. Buu decides to expel his evil half. Evil Buu is born and defeats the now Good Buu, absorbing his power. Super Buu is formed. Vegeta then tells the other Z-Fighters to escape and uses all of his power to defeat Buu. Vegeta dies, and Buu seemingly does as well. Unfortunately, Buu reforms his body and begins his hunt for the Z-Fighters.
Super Buu heads to the Lookout, where the Z-Fighters and their families are currently. Goku returns to the Otherworld. They have trained in the Room of Spirit and Time. During this time, Raditz, Gohan, Tomata, Trunks, Goten, and Daika have increased their power level. Goku taught them the Fusion Dance, which they have been trying to master. Piccolo temporarily distracts Buu by saying he should go kill humans for a while, banking on using the Dragon Balls to bring them back. Super Buu then kills the whole population of Earth with one attack. This takes just long enough for Trunks and Goten to come out of the chamber, having mastered fusion. They become Gotenks and challenge Buu. A fight begins to take down Buu. Sensing how strong Buu is, Raditz and Gohan fuse, becoming Rahan.
With this new power, the Z-Fighters manage to beat down Buu, but his regeneration keeps him from being defeated. Super Buu ends up absorbing most of the Z-Fighters except for Goten, Trunks, Daika, and 17, who manage to escape. With the absorbed fighters, Buu becomes several thousand times more powerful, becoming Buahan.
Meanwhile, in the Otherworld, Goku and Vegeta see the chaos unfolding. They ask if there is a way for them to return to the Earth to help their friends. The Supreme Kai tells them they can if there is no hope for the universe. With the absorption of Rahan, they are left with no choice. Before they leave, the Supreme Kai gives them the Potara. He tells them that using these earrings will allow them to godly fuse, multiplying their power, but they will have to remain fused. Goku and Vegeta, now on Earth, sense Buu’s immense power. They get smacked around by Buu until they decide the only way to save the world and their families is to fuse. They fuse, becoming Vegito.
Base Vegito toys with Buahan for a while, lowering his power to around Buahan’s. Buahan, convinced with his superiority, then absorbs Vegito. Right before he is absorbed, Vegito goes to full power and goes Super Saiyan 3 for the first time. This power decimates Buahan and all of the absorbed people are released. The loss of his good side causes Buu to go back to his original form: Kid Buu.
The explosion causes the Potara earrings to shatter, separating Goku and Vegeta. Gohan wants to go help, but Shin and Kibito Kai come and meet him and the others. They tell Gohan they are here to help and Gohan goes with them to unlock his potential.
During the 26 hours it takes to unlock Gohan’s ultimate potential, Goku and Vegeta struggle to protect the others. Although their combined power is much more powerful, Buu seems to have almost infinite power and regeneration abilities. With difficulty, he manages to stall out Buu until Gohan returns.
Gohan has now unlocked his Mystic Form. This form unlocks his full potential. Returning to Earth, Gohan finds the remaining fighters on the ropes. Daika, Trunks, and Goten have achieved Super Saiyan, but even that isn’t enough to stop the menace.
Before Kid Buu can absorb them, Gohan steps in. Gohan manages to overcome Buu’s power. Gohan, Goku and Vegeta use the last of their power to use a combined Special Beam Cannon/Final Flash/Kamehameha. Kid Buu is finally killed. Using the Dragon Balls, they bring back everyone who died to Buu’s rampage. Good Buu remains on Earth, finally fully cleansed of his evil side. Kid Buu is reincarnated as Uub, keeping his latent power but becoming human and good.