Seven years have passed since the battle with Cell. Gohan is now a high school student attending Orange Star High School, in Satan City. Gohan also has a younger brother named Goten, who was born after Goku died. On his way to his first day of high school, he comes across a bank robbery in the city and stops it with his Super Saiyan form. As rumors spread about a "Golden-Haired Warrior", Gohan gets to know his new classmate and Mr. Satan's daughter, Videl. After a day of trying not to act suspicious in front of his classmates, Gohan travels to Capsule Corporation to talk to Bulma about his problem.

Gohan approaches Bulma, who gives him a transformation watch to hide his true identity. On his way home, Gohan encounters two people driving recklessly. This is where he comes up with the alias "Great Saiyaman", and scares the people into driving more safely on the road. Gohan returns home later that night, where Chi-Chi and Gohan's little brother, Goten, discover that Gohan now has a costume to conceal his identity while fighting crime. The next day, Videl is called in to stop a bus hijacking and handles the criminals, but the bus falls off a cliff, as nobody is driving the vehicle. Gohan uses the alias \"Great Saiyaman\" once again and pitches in to save Videl and the passengers.

Gohan starts her off with the basics of learning how to control her energy. Within a day, Videl has managed to learn how to float, while Goten has managed to learn how to fly. Following a remark from Gohan, Videl comes back the next day with her hair cut. Videl is determined to learn more from Gohan. Meanwhile, Vegeta is also surprised to find that Trunks is also capable of becoming a Super Saiyan, whose strength is slightly higher than Goten's. Goku trains fiercely in anticipation for the tournament.

After ten days of training, Videl manages to master flight. This finally gives Gohan and Goten the opportunity to resume their training for the tournament. On the day of the tournament, Gohan suggests to Vegeta, Goten, and Trunks that they do not use their Super Saiyan forms during the tournament in order to avoid hassle from the press. Arriving at the island where the tournament will take place, Gohan and the others are greeted by Goku, whom Fortuneteller Baba had granted 24 hours visit to Earth. Goku meets Goten for the first time, while Piccolo destroys all the cameras in the area to keep Gohan's identity a secret. After the participating fighters register for the tournament, Goten and Trunks are forced to participate in the juniors division.

While Gohan and Videl wait to take their preliminaries, Goku, Vegeta, and the others go to spectate the junior division, where Goten and Trunks effortlessly progress through each round until they face each other in the final round. The battle gets more intense. Goten and Trunks surprise the audience with advanced techniques. The fight between Goten and Trunks continues.

During an intermission, Trunks and Goten decide to take out one of the participants and steal his clothes. Trunks and Goten take his place in the adult division. As Goku and the others get something to eat, they encounter a pair of mysterious participants named Shin and Kibito, who Piccolo determines as not being from Earth. As the sixteen qualifying participants gather together to decide the drawings for the first round, Gohan is paired up against Kibito. Goku gets paired up against Vegeta.

As the drawings come to a close, Piccolo is put up against Shin. Android 18 is put up against Mr. Satan. After Krillin wins his first match against his opponent, Piccolo and Shin face off against each other. As Piccolo senses an overwhelming power coming from Shin, he decides to forfeit the match before it even begins. Piccolo feels Shin's power is far beyond his own. Videl prepares to go up against a man named Spopovitch.

With Spopovich nearly beating Videl to death, Gohan loses his temper and prepares to storm onto the field, but Spopovich's partner, Yamu, convinces Spopovich to end the fight by pushing Videl out of the ring. With Videl heavily injured, Goku uses his Instant Transmission to deliver some Senzu beans from Korin, which Gohan gives to Videl to restore her to full health. Gohan, whose identity is revealed to his classmates after his bandana was blown off during his outburst, steps up to face against Kibito, who requests to see Gohan's Super Saiyan transformation. After Piccolo informs the other Z-Fighters of Shin's identity as the Supreme Kai, he orders Gohan to transform. Right after Gohan transforms, Spopovich and Yamu appear and use a device to drain his Super Saiyan energy, leaving him weakened. As Kibito offers to tend to Gohan, the Supreme Kai asks Goku and the others to join him in pursuing the culprits.

As Goku and the others set off to join the Supreme Kai, Kibito uses his unique ability to restore Gohan's energy, before the two of them, along with Videl, set off after the others. Along the way, Supreme Kai and Kibito explain to the others about how an evil wizard named Babidi is seeking to revive an evil being known as Majin Buu, who was responsible for the death of four Supreme Kais long ago. To accomplish this, he manipulated Spopovich and Yamu to join the tournament and steal Super Saiyan energy needed to break the seal placed on Buu. Unable to push her flight any further, Videl, coming to understand that Gohan was the one who defeated Cell back then, places her faith in him and returns to the tournament grounds, while Gohan and Kibito catch up to Goku's group. Arriving at the site of a buried spaceship, the Z-Fighters lie in, watch as Spopovich and Yamu deliver Gohan's energy to Babidi and his partner, Dabura, the Demon King.

Supreme Kai and Kibito warn Goku and the others about the malicious pair, with Dabura being the strongest being of the underworld and Babidi possessing magic powerful enough to sway him to become his henchman. Having obtained the energy from them and deeming them of no further use, Babidi kills Spopovich with his magic while having his other henchman, Pui Pui, kill Yamu. Having detected the presence of the Z-Fighters, Babidi and Pui Pui head inside of the ship while Dabura attacks the group, immediately killing Kibito and firing his spit at Krillin and Piccolo, turning them into stone. Learning that they can only unpetrify them by killing Dabura, the remaining fighters chase after him into the ship, despite Supreme Kai's warnings. Upon reaching the first floor, they are confronted by Pui Pui, who informs them they must face several floors before they can reach Babidi at the bottom.

As Vegeta easily outmatches Pui Pui in terms of fighting skill, Babidi changes the stage to match that of the high gravity planet, Zun, where Pui Pui is at his top fighting form. However, this makes no difference to Vegeta, who obliterates his opponent and opens the way to the next floor. Believing that the Saiyans should not be underestimated, Babidi sends in his next warrior, Yakon, and changes the stage to that of Planet Ankoku, but even with the aid of the pitch-black darkness, Yakon is no match for Goku's keen senses. When Goku turns Super Saiyan, however, Yakon uses his ability to consume light and sucks in Goku's energy, reverting him into his normal state. Rather than finding an alternative way to fight, Goku instead feeds him even more Saiyan energy, until he explodes from being overloaded, allowing Goku and the others to move onto the next stage.

As Babidi fears the power of the Saiyans, Dabura decides to face them himself, going inside a meditation chamber to raise his power before his battle. Meanwhile, back at the tournament, Goten and Trunks take a break from their Mighty Mask disguise to get some drinks, barely avoiding having their cover blown. With Goku and the other members still missing, Mr. Satan suggests that a battle royale be held between the remaining contestants; #18, "Mighty Mask", Killa, Jewel, and himself. As the match begins, Killa and Jewel are quickly dispatched by #18 and the Trunks/Goten combo, who proceed to ignore Mr. Satan and go after each other.

As the battle between #18 and \"Mighty Mask\" continues to escalate, #18 struggles to keep up with her opponent's unpredictable moves, but manages to land a critical hit against the pair, knocking them into the center of the stage. Goten and Trunks decide to switch places with Goten now on top, pushing #18 to the point where she starts using energy blasts against them, but she manages to hit back with support from her daughter. The pair decide to go Super Saiyan and launch a powerful blast at #18, but she manages to dodge it and reveal their true identity, resulting in the pair becoming disqualified, leaving #18 and Mr. Satan as the only remaining fighters. As Goten and Trunks come across Videl upon her return, Dabura emerges from his meditation chamber, ready to face off against Gohan.

Noticing his complete lack of fighting skill, #18 makes a deal with Mr. Satan to let him win the match and maintain his dignity in exchange for twice the price money. Meanwhile, Goten and Trunks, having heard about the current situation from Videl, become interested and head towards the action. Back at the spaceship, Dabura appears before Gohan, deciding to give him a handicap by choosing an arena similar to Earth's environment. As the fight gets underway, both Gohan and Dabura bring out their full power against each other. Despite being hit by some of Dabura's powerful attacks, Gohan refuses to lose against him.

As the battle between Gohan and Dabura continues, Vegeta's impatience to settle things with Goku catches the attention of Dabura, who decides to abandon the fight and report to Babidi. Learning that Vegeta still retains a lot of evil in his heart, Babidi uses his magic to target Vegeta's mind and sway him over to his side. To make matters worse, Babidi transports everyone to the tournament grounds, where Vegeta, still focused on his desire to defeat Goku, prepares to face off against him, even with innocent bystanders watching.

Vegeta begins his attack on Goku, killing several innocent people in the crowd without any remorse. Realizing that Vegeta had fallen under Babidi's spell on purpose, purely for the sake of his grudge, Goku is left with no choice but to fight against him, despite Supreme Kai's attempt to stop him. Goku also has Babidi warp them to someplace else where they can fight alone. As Supreme Kai and Gohan decide to go on ahead and leave the two Saiyans to their fight, Babidi attempts to command Vegeta to kill them, but his pride defies even his master's demands.

While Goku and Vegeta's fight gets underway, building energy for Majin Buu each time Goku takes damage, Gohan and Supreme Kai arrive at Majin Buu's egg, where they are confronted by Babidi and Dabura. Before the fight between them can begin, Majin Buu's egg reaches full power, as a result of Goku and Vegeta's battle.

Goku and Vegeta's fight continues to escalate, with Vegeta revealing he chose to go under Babidi's control in order to gain the strength to face against Goku, feeling he needed to get rid of his gentle side. Meanwhile, as the seal containing Majin Buu starts to be break, Gohan attempts to stop him reaching full power by shooting multiple Kamehameha Waves at the egg. The egg splits apart with seemingly nothing more than pink smoke coming out of it, leading Supreme Kai to presume Majin Buu had been destroyed. However, Gohan senses a powerful energy from the smoke, which forms together to make Majin Buu, who appears to be nothing more than a mere child.

Everyone seems to be bewildered by Buu's childlike behavior, curious as to whether his rebirth was actually unsuccessful. However, Buu soon shows off his frightening strength when he effortlessly beats Dabura, the rise in his power level even catching the attention of the other Saiyans. Although Goku tries to convince Vegeta to postpone the fight in order to stop Buu, Vegeta knocks out Goku when he drops his guard, taking Goku's senzu bean and setting off to fight Buu himself. Gohan and Supreme Kai attempt to flee from Buu, but he easily catches up to them, proving himself immune to Supreme Kai's attacks.

Buu completely overwhelms Supreme Kai and Gohan, with Supreme Kai using the last of his strength to make sure Gohan survives Buu's attack. Before Buu can finish Supreme Kai, he is attacked by Dabura, who believes he cannot be made to obey anyone. Meanwhile, Goten and Trunks arrive nearby, inadvertently breaking the petrified Piccolo. Unfazed by Dabura's attacks, Buu uses his unique ability to turn Dabura into a cookie and eats him, undoing the petrification on Krillin and Piccolo, the latter of whom survived being broken thanks to his regenerative abilities. Before Buu can do the same to Supreme Kai, Vegeta arrives on the scene.

Vegeta, willing to put his life on the line, begins his assault on Buu, managing to blast a hole through him. However, Buu manages to regenerate himself and builds up his anger to create a giant explosion, dealing a great amount of damage to Vegeta.

As Vegeta is ensnared by Buu, Trunks and Goten can no longer stand by, and rush to the scene to rescue him, while Piccolo attacks the defenseless Babidi and rips him to pieces. Feeling he cannot defeat Buu with conventional means, Vegeta says his farewells to Trunks before knocking both children out, instructing Piccolo to take them far away. Once everyone is cleared from the scene, Vegeta uses his last resort, overloading his Super Saiyan form and sacrificing himself in a massive self-destructive explosion, in order to obliterate Buu.

As Bulma and the others from the stadium follow Videl's directions in pursuit of Goten and Trunks, they are hit by the shockwave from Vegeta's attack and almost crash, but are saved by #18. Piccolo leaves Goten and Trunks with Krillin, telling him to inform the others that Vegeta and possibly Gohan have been killed, before he investigates the site of Vegeta's sacrificial attack, finding various charred fragments of Buu, along with Babidi, who narrowly survived the explosion. However, Piccolo is shocked to witness Buu's fragments suddenly come to life and reassemble, to bring Buu back to his normal form. As Buu uses his power to heal Babidi, who vows to make the planet suffer, Piccolo decides to take Goten and Trunks up to Kami's tower. Elsewhere, Goku regains consciousness and tries to determine the situation, while a weakened Supreme Kai searches for Gohan, whose fate is still unknown.

Bulma's group goes to seek out the Dragon Balls so they can revive all the spectators that Vegeta killed, coming up against a large dragon whilst trying to get the last one. Meanwhile, as Piccolo and Krillin as they leave Goten and Trunks in the care of Dende and Mr. Popo, they are soon joined by Goku, who learns about the situation. Goku states he isn't strong enough to defeat Buu, but mentions having learned a technique known as Fusion, in which two similarly built fighters can merge together to become a single powerful fighter. Though he had originally planned to do this himself with either Gohan or Vegeta, Goku realizes from Mr. Popo that he can instead teach this technique to Goten and Trunks, due to their similar size.

Using the Dragon Balls, Bulma summons out Shenron, using one of its wishes to bring back everyone who had been killed that day. As using all three wishes would mean they would have to wait another year before they can use the Dragon Balls again, Goku warps over to Bulma's group and convinces Shenron to postpone the other two wishes, shortening that time to four months. Kibito, who was revived by the wish, manages to find Supreme Kai and heal his injuries, and together they manage to find Gohan and decide to bring him to the world of the Kais. Meanwhile, Goku warps Bulma and the others to Kami's Palace, where he informs them of the grim situation concerning Gohan and Vegeta, before Babidi announces his message to the world.

Babidi announces that he will have Buu destroy the planet unless Piccolo, Goten, and Trunks are brought to them within five days. Piccolo almost sets off but Goku convinces him to stay, saying they can bring back everyone who dies with the Dragon Balls. Meanwhile, Gohan, Supreme Kai, and Kibito arrive in the world of the Kais, where Supreme Kai reveals he wants Gohan to master the legendary Z-Sword in order to defeat Buu. Back at the palace, Goten and Trunks wake up and are given a crash course in Fusion by Goku.

Using his Super Saiyan strength, Gohan manages to pull the Z-Sword from its resting place, finding it to be incredibly heavy. Meanwhile, Babidi makes another broadcast of Buu laying waste to another city, prompting Trunks and Goten to contact him, informing him that they will soon grow strong enough to defeat them. The pair soon begin their Fusion training, learning how to match their Super Saiyan energy with each other. Meanwhile, Babidi learns where Trunks lives, and announces his plans to attack Capsule Corporation in Metro West.

Realizing that they won't be able to find the Dragon Balls if the Dragon Radar is destroyed, Goku orders Trunks to head to West City to retrieve the Dragon Radar from Capsule Corporation, while he heads off to delay Buu and Babidi. In order to buy Trunks some time, Goku builds up a large amount of energy, which also uses some of his remaining time on Earth, to transform into his ultimate form, Super Saiyan 3, with its immense power being felt even in the world of the Kais.

As Goku begins his battle against Buu, who demonstrates an ability to copy other people's techniques, Trunks arrives at Capsule Corporation, but struggles to find where the Dragon Radar is. Bulma soon realizes that she had left the Dragon Radar in her aeroplane, and manages to inform Trunks about it. Sensing Trunks has accomplished his mission, Goku undoes his Super Saiyan 3 transformation once more, and takes his leave after informing Buu that Trunks and Goten will be ready to fight him in two days. Shortly after Goku leaves, Buu, deciding he has no further use for Babidi, turns against him and kills him.

Without Babidi to order him around, Buu continues to freely destroy cities while waiting for his opponents. With less than thirty minutes left on Earth due to the energy spent in Super Saiyan 3, Goku wastes no time teaching Goten and Trunks the key to the Fusion technique, the somewhat embarrassing-looking Fusion Dance.

Buu lays waste to another village, turning its citizens into clay in order to build himself a house for a brief rest. Meanwhile, Goten and Trunks, slightly bored from their training, ask Goku to show them his Super Saiyan 3 transformation. However, upon doing this, Goku uses up the remainder of his time on Earth. After saying goodbye to everyone, Goku returns to the afterlife, leaving the Fusion training in the hands of Piccolo. Meeting up with Yemma, Goku is relieved to hear that Gohan isn't dead and decides to follow his energy trail.

Goku arrives in the Realm of the Kais where Gohan is training with the Z-Sword, deciding to help him with his training. Meanwhile, Buu, who has already killed two thirds of Earth's population, comes across a blind young boy and heals his vision, becoming surprised when he thanks him instead of being afraid. With Gohan growing more proficient with the Z-Sword, Goku tests out the sword's sharpness by throwing a metal cube at it, only to inadvertently break the sword in half instead. However, from the broken sword emerges the Supreme Kai of fifteen generations ago, Old Kai.

After some dubious persuasion from Goku, Old Kai reveals he has the ability to draw out someone's true power beyond their limits, though this turns out to be a lengthy process that can take over a day to complete. Meanwhile, Goten and Trunks try out the Fusion Pose for real, but an error in the Fusion Pose results in the merged fighter, Gotenks, ending up with a failed transformation. After another failure, the pair eventually perform a successful fusion and transform into the true Gotenks who, confident that he can defeat Buu with his current power level, goes off in search of Buu, despite Piccolo's warnings.

Gotenks arrives to fight against Buu but, despite showing a great amount of strength, is overwhelmed by his power, barely managing to escape from him after he launches an all-out attack against the armed forces. Meanwhile, Mr. Satan reluctantly confronts Buu, who becomes amused with his traps disguised as presents, and decides to make him his servant.

After completing his ceremony, Old Kai instructs Gohan to sit still for the next 20 hours in order to complete the power-up. Meanwhile, Goten and Trunks learned how to perform the fusion while in Super Saiyan form, becoming Super Gotenks. Gotenks' cockiness almost leads him to confront Buu again, but ends up retreating when the Fusion time runs out. Elsewhere, Buu inadvertently befriends a small dog after healing its injured leg, while Mr. Satan almost manages to convince Buu to stop killing humans. However, a hunter looking to take down Buu for himself relentlessly shoots the dog, leaving Buu shocked and infuriated.