This story begins at a picnic, during a painful singing session courtesy of
Krillin, when a massive ship arrives. Out comes
Paragas, one of the few surviving Saiyans. He invites Vegeta to rule a new
Planet Vegeta. Vegeta initially refuses until Paragas appeals to Vegeta's ego by asking him to kill the
Legendary Super Saiyan that has completely destroyed the south galaxy.
Gohan, Trunks, Krillin, Master Roshi and Oolong depart with Vegeta.
Meanwhile, Goku is at a school interview with Chi-Chi, who is trying to get Gohan into the school. Goku is summoned telepathically by King Kai during the interview, and immediately teleports to him upon hearing the south galaxy has been destroyed, leaving the school staff confused and Chi-Chi very annoyed.. King Kai had sensed the Legendary Super Saiyan in the south galaxy so he sent Goku to stop him. By the time Paragas had arrived, Goku had already been searching for the Legendary Super Saiyan.
At first the group believes that Paragas has good intentions and honestly wants to create a new Planet Vegeta. Later on it is revealed that the planet is on a collision course with a large comet, Comet Kimori.
Soon after Gohan, Trunks, Krillin, Roshi, Oolong, and Vegeta arrive, a report comes in that the Legendary Super Saiyan is attacking another planet. Vegeta takes Paragas's son, Broly, and investigates.
While Vegeta and Broly are investigating the attack, Gohan, Trunks and Krillin go out to investigate the new Planet Vegeta. They find a slave colony strip mining the planet to send energy to the palace. Gohan, Trunks and Krillin quickly break up the slave labor going on at the colony. This is also when Goku arrives on the planet to be accidentally punched by Krillin. Paragas also first meets Goku during this scene, and immediately identifies him as Bardock's son.
The first time that Broly sees Goku he immediately recognizes him and gets extremely infuriated. Broly and Goku begin powering up in a standoff, until Paragas brings him back under control by using a mind-control device put on him when he was a teenager. Unfortunately for Paragas, Broly has found a way to bypass the mind-control which the device has over him.
During this scene we are told, by Paragas, that Goku and Broly were born on the same day. Broly was born with an extreme fighting power of 10,000 while Goku was born with a weak power level of 2. This is quite unusual since a grown human male has a power level of 5. It is said that Goku cried constantly, keeping Broly awake for days on end. From then on Broly would constantly hold a grudge against Goku.
After Paragas reflects on the past, Broly attacks Goku during the night. It is in this fight that we see Broly in his restrained mastered Super Saiyan form. Towards the end of the fight Paragas reasserts control over Broly and forces him to stop attacking Goku. He barely manages to do so, foreshadowing things to come.
Eventually Gohan, Trunks and Krillin bring the slaves they freed back to the palace, and they identify Broly as the one that attacked their planet. Broly walks towards Goku shouting, "Kakarrot!" repeatedly. Broly goes into his restrained mastered Super Saiyan form again while in the walk towards Goku. Vegeta transforms into his Super Saiyan second grade form and attacks Broly with a Big Bang Attack but the attack has no effect. Then Paragas decides to reveal the entire story. Broly soon rages out of Paragas's control and becomes a monsterous behemoth of a Super Saiyan, his true and most powerful form. Vegeta, stunned by Broly's power, falls to the ground in shock, eyes wide with fear, realizing that Broly is the feared Legendary Super Saiyan.
Broly, raged beyond control, destroys the strip mines and the slaves' home planet. Broly goes after Goku and Gohan in a chase, destroying forests and slaves with a barrage of ki blasts. The chase stops and Goku turns into a mastered Super Saiyan. Gohan follows suit and then Trunks comes to help and transforms into his Super Saiyan second grade form. The battle gets intense, but remains completely one-sided. Goku tells Gohan to run, which he does, but Broly valiantly pursues him and smashes him into a building, from where he hangs by the cloth around his neck. The destructive Super Saiyan then single-handedly takes down both Goku & Trunks. Then Piccolo shows up after Broly annihilates the boys and saves Gohan from a would-be-fatal attack. Piccolo then gets around to handing out everyone senzu beans. Not long after, the four warriors then decide to gang up on a patient Broly, who stands on a thin cliff. In only a matter of scenes, all four of them become annihilated by Broly again. Piccolo attempts to get Vegeta to fight. In shock and pride, he refuses, and is abandoned by Piccolo. Meanwhile, Goku is having a hard time doing anything to Broly. Vegeta, rediscovering his inner Saiyan pride, transforms into his Super Saiyan second grade form once again, and joins the fight against Broly. His efforts, however, are futile as he is beaten without mercy. Broly later finds his father, Paragas, attempting to flee inside a Saiyan space pod, leaving his son to die in the explosion of the planet. Broly picks up the pod in turn and crushes it, killing Paragus, and throws it directly at the comet.
Piccolo gets the idea for everyone to channel their full powers to Goku so he can defeat Broly. One by one they comply, except Vegeta, who refuses to help a lower-ranking Saiyan due to his pride. While gathering energy, Goku suffers at the hands of Broly; he is blown across the landscape by ki blasts and very powerful punches, and then repeatedly jumped on. Finally, on the brink of defeat, Goku convinces Vegeta to help him. At first, Vegeta still refuses, but then reluctantly does so, saying "This still doesn't change anything." and with the combined power of the Z-Warriors, Goku is finally able defeat Broly by blasting him into the comet. Everyone then escapes from the planet via a Caspule Corp. ship which Piccolo used to reach them. Goku and Gohan teleport home, only be confronted by an angry Chi-Chi, who is still annoyed at Goku for leaving during the interview and then passes out