Goku is competing in a fighting tournament of the dead. His meets a stalement with his new friend and opponent, Paiku-han. On Earth, Gohan is training Goten and Vegeta is training Trunks. Gohan and Vegeta become shocked at the fact that Goten and Trunks can reach Super Saiyan stage at such young ages. There is another Tenkaichi Boudoukai on Earth. Goten and Trunks want to attend in the competition, but are banned because they are too young. However, they steal a competitor's costume and wear it. They battle Android 18 and their disguise is revealed. Piccolo faces someone known as Kaiou-shin, but freezes upon sensing his opponent's power. Goku is granted a single day of life to fight at the tournament so he is also present. It is Gohan's turn to compete. He has assumed the identity as a masked crime fighter known as the Great Saiya-Man. Gohan powers up and turns Super Saiyan mode. This blows off his disguise and the audience learns that the Great Saiyan-Man is really the nerd Gohan. Spopobitch and Yamu, two competitors with a "M" on their foreheads, fly out to the battle ring and approach Gohan. Then, the two mysterious men take out a lamp and use it to absorb Gohan's energy. Gohan falls unconsicous, Spopobitch and Yamu fly away, and everyone else become shocked and wonder what just happened. Kaiou-shin and his partner approach the Z warriors and revive Gohan. Then, they tell their story. They are really godly beings beyond Kaiou and are here to stop the plot of Babi-di and Darbura. Babi-di is an ugly little being who wants to destroy all life just for the fun of it. Darbura is the Prince of Darkness. Babi-di and Darbura are allied at the task of reviving someone called Buu. To do so, a lot of energy is needed. Everyone learns that Spopobitch and Yamu are zombies of Babi-di. That is why they absorbed Gohan's energy. Allowing Sopopobitch and Yamu to attack Gohan was part of the plan to follow them to Babi-di's lair. A new terror breaches over the Earth. The Z warriors decide to help Kaiou-shin for the sake of their planet, and follow Spopobitch and Yamu to Babi-di and Darbura's lair. Babi-di and Darbura's minions are killed in the battles to come. Our heroes face Darbura now. Unaware of Darbura's ability to "stone", Krillen and Piccolo are turned into stone. The only remedy to the curse would be Darbura's death. Darbura is eventually killed and Krillen and Piccolo are restored to normal.

Learning that becoming Babi-di's slave will make him stronger, Vegeta volunteers to become Babi-di's slave. A "M" appears on Vegeta's forehead and he turns evil once again. Majin-Vegeta looks forward for another battle against Goku and hopes to defeat him once and for all. Vegeta's power donation to Babi-di grants enough energy to revive the fat being known as Buu. Buu is re-awakened and Babi-di rides around the city on top of him. Cities begin to get terrorized. Majin-Vegeta knocks out Goku, but realizes that good triumphs over evil. He regains his sanity and truely becomes good. Majin-Vegeta goes off for Buu and fights him. Buu is a lot stronger and has the advantage. In a final effort to defeat Buu, Vegeta self-destructs, hoping that his sacrifice will take Buu down with him. To his dismay, Buu survives. Even in Super Saiyan level 2, Goku is unable to defeat Buu. Rage increases, and Goku reaches Super Saiyan level 3 for the first time! Meanwhile, Bulma gathers the dragonballs to revive all the civilians killed by the Majins since the Tenkaichi Boudoukai. Buu grows tired of being Babi-di's slave and kills him. Then, he purifies himself by fission and splits into two beings. One is pure good, and the other is pure evil. The two Buus fight and the evil one turns the good Boo into a cookie. The cookie is eaten, and Boo powers up in a body transformation. The fat Buu is now leaner and more muscular.

Meanwhile....
Gohan heads to Kaiou-shin's planet to receive training. As a sign of respect, he wears the Kaiou-shin suit. Gohan pulls out the legendary Z Sword and begins to train under Dai Kaiou-shin, the Elder Kaiou-shin. Gohan is learning how to use his hidden powers without getting angry to go Super Saiyan mode. Kaiou-shin reflects at his past. When he was young, he lived in peace with his family at his home planet. Then, an evil sorceror, Babi-di's ancestor, used Buu to terrorize. (At the time, Buu was smaller than how he appears during the current section of the series.) Buu killed all of Kaiou-shin's family members. The Kaiou-shins cursed Buu into his fat form and made him dormant. Kaiou-shin, his partner, and Dai Kaiou-shin are the only Kaiou-shins left. Now, years later, Buu has been revived.

Everyone is at Sky Palace (Dende and Popo's palace) except Gohan, who is still training under the Kaiou-shins. Goku taught Goten and Trunks the art of fusion. Goten and Trunks fuse and become Gotenks! Then, he teleports to Kaiou-shin's planet and watches everything through Kaiou-shin's crystal ball. Buu heads for Sky Palace to look for the "strong" being he sensed. He needs to absorb more powerful beings to be restored to his original form, the stronger form that he was in before the dying Kaiou-shins cursed him. The "strong" being in which Buu searches for is Gotenks. Goten and Trunks are training in a the room of spirit and time inside Sky Palace. Piccolo tricks Buu into waiting for the boys to come out. Time passes by slower in the room, so it will buy some time for the boys to train more. Chi-Chi, seeing Buu, goes up to him to scold him for killing so many innocent people. Buu turns Chi-Chi into an egg and squashes it. Buu grows tired of waiting and threatens to be taken into the room in which Goten and Trunks are in, or else he will kill everyone at Sky Palace. Piccolo, knowing that he is too weak to fight Buu, takes him to Goten and Trunks with the hope that the boys have become strong enough to fight Buu. Goten and Trunks fuse into Gotenks and fight Buu. Gotenks is able to stand up to Buu, but neither side is winning. Piccolo blasts the doorway in the room. Then, he tells that without the exit, everyone will remain trapped in it forever so Buu can no longer kill any more people. Gotenks argues with Piccolo that they too have becomed trapped. Buu creates a fireball that produces a portal exit. He exits the room and returns Sky Palace. Gotenks and Piccolo look at each other in grief; they are now trapped and Buu is free. Super Saiyan Gotenks tries to create the same fireball that Boo created, but is too weak. He powers up and reaches Super Saiyan level 3. Now much stronger, he is able to produce the portal. Piccolo and Gotenks exit the room and face Boo once again.

In the battle followed, Sky Palace is destroyed. Super Saiyan level 3 Gotenks is overwhelming Buu and winning. Buu is thrown back to Earth. Gotenks and Piccolo return to Earth to search for Buu's body. Buu survived the attack and reappears. Gotenks prepares to fight again, but the fusion wears off. Goten and Trunks unfused are too weak to fight Buu. Gohan completes his training with Dai Kaiou-shin. He chooses a change in clothes to wearing his father's suit and wishes everyone goodbye. Goku wishes his son luck, but envies that he cannot help because he is dead. Kaiou-shin's partner teleports Gohan to Earth. Gohan appears in time to save Goten, Trunks, and Piccolo. Gohan beats Buu and Buu self-destructs. Thinking they won the battle, Gohan, Goten and Trunks fly off. They see Satan and Dende on their journey and take them with them. A being flies towards them. Its Buu! Goten and Trunks fuse again and are in Super Saiyan level 3. Eagerly wanting to fight again, Gotenks boasts about beating Buu. Taking advantage of the distraction, Buu absorbs Super Saiyan level 3 Gotenks and undergoes another body change. Now a lot stronger, Buu overwhelms Gohan. Buu throws a fireball at Gohan. Gohan tries to block, but another fireball pushes Buu's fireball away from Gohan. The ally is Tenshin-han. However, Boo prepares another fireball. In the judgement court, King Yemma grants Vegeta life so that he could help save the planet. In Kaiou-shin's planet, the Kaiou-shins tell Goku about Pottara Earrings. Kaiou-shin and his partner demonstrate. Each partner wears one of the earrings and the to permanently fuse. The earrings are given to Goku. He is to give one to Gohan so that he and Gohan could fuse to beat Buu. To bring Goku back to life, Dai Kaiou-shin exchanges his life for Goku. Goku's halo disappears, and Dai Kaiou-shin falls dead. Then, he re-awakens with a halo on top of his head. Goku thanks everyone and teleports to help save Earth.

Buu is about to throw a huge fireball at Gohan. Goku teleports to the scene and throws a fireball at Buu. Buu is sliced and his legs and lower part of his antennae are cut off. Goku tells Gohan about the fusion earrings and throws one to Gohan. Gohan misses it and the earing fell inbetween some rocks. After finds it. The cut off part of Buu's antennae engulfs Piccolo and Buu changes into a stronger form again. The cut off lower body of Buu walks and kicks Tenshin-han unconscious. Then, it engulfs Gohan and Buu powers up again. Goku feels despair again. Vegeta appears and aids Goku. Goku tells Vegeta about the fusion earrings and Vegeta finds the earring dropped by Gohan when Gohan got absorbed. Goku and Vegeta are pulled together and become Vegetto! Vegetto beats Buu, but purposely gets absorbed by Buu to save their comrades. The absorption cancelled out the effect of the permanent fusion and Vegetto splits back into Goku and Vegeta. They find and rescue Goten, Trunks, Gohan, and Piccolo by carrying the bodies and escaping through one of Buu's pores. Good Buu is also still in Buu's body. As the characters left Buu's pores, they returned to their original size. Meanwhile, absorbing Vegetto had given Buu enough energy to complete his phase change. He powers up as the escapes from his body takes place, and returns to his original form. "Original" Buu is back. Buu shoots a giant fireball and blows up the Earth. The newly fused Kaiou-shin teleports to help Goku teleport Vegeta, Trunks, Goten, Gohan, Piccolo, Dende, and Satan to safety in Kaiou-shin's planet. Buu teleports and follows Goku. Since goku didnt want destroy the planet, Goku, Buu and Vegeta teleport to another planet. Goku and Buu battle. Goku gets beaten and is weak, so Vegeta joins the fight. Vegeta admits that Goku is better than him for the first time and then fights Buu. Buu beats Vegeta and is about to kill him, but Satan pops up and threatens Buu. Buu attacks Satan but Satan dodges the punch. All of a sudden, Buu goes in pain and spits the good fat Buu out. The two Buus fight and the good Buu gets beat unconscious. Goku and Vegeta continue to face Boo.

Vegeta tells Kaiou-shin to gather the Namek dragonballs. He does so, and makes the first wish to restore the Earth. The second wish revives all the civilians that Buu killed. The third wish is saved for later. Everyone is back on Earth except Goku, Vegeta, and Buu. Goku begins to create a Genki Dama while Vegeta distracts Buu. Our heroes on Earth tell people to raise their hands. Doing so will donate energy to Goku's fireball. However, no one listens. Satan, still famous after the Cell Game, raises his hands. Following the example of their idol, everyone on Earth raises their hands. Goku gets enough energy for his fireball and he fires it. Buu is too strong to get blasted and holds the fireball back. Kaiou-shin makes the last wish from the dragonballs to restore Goku's energy. Goku's power restores from his weakened state and he uses his revived strength to strengthen his fireball. Buu, unable to repel Goku's Genki-Dama, is killed by the fireball. Our heroes have won the battle. Fat Buu awakens. Our heroes debate on whether or not to kill him, but Satan pleads that this Buu saved his life. Buu is spared and joins the Z warriors.

Ten years later...
Goten and Trunks are teenagers. Gohan married Videl, and the two became parents of five-year old Pan. Goku wished the evil Boo back to life to fight him again. The wish reincarnates Buu into Ubuu; all evil cleansed. Goku is training Ubuu. There is peace on Earth...