This story begins at a picnic, during a painful singing session courtesy of Krillin, when a massive ship arrives. Out comes Paragus, one of the few surviving Saiyans. He invites Vegeta to rule a new Planet Vegeta. Vegeta initially refuses until Paragus 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 on Paragus’ spaceship (with Trunks’s intention being to try and stop his father from doing something foolish).
Meanwhile, Goku is at a school interview with Chi-Chi, who is trying to get Gohan accepted into a good school. Goku is reached telepathically by King Kai during the interview, who tells him that there is very urgent news. Goku then teleports to King Kai’s planet, leaving the school staff confused and Chi-Chi very annoyed. King Kai then tells Goku that a Super Saiyan is destroying planets in the South Galaxy and asks Goku to step in and stop him.
At first the group believes that Paragus has good intentions and honestly wants to create a new Planet Vegeta (though Future Trunks is suspicious about the whole situation). Later on it is revealed that the planet is in the path of a gigantic comet, “Comet Camori”, which is much larger than the new Planet Vegeta. 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 Paragus’s son, Broly, and investigates. Meanwhile, Goku teleports to a planet already ravaged by the Legendary Super Saiyan, and Goku feels the echoes of the enormous ki that was present. Goku, having not seen the Legendary Super Saiyan, teleports to the new Planet Vegeta.
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. At this point Goku arrives on the planet and is accidentally punched by Krillin. Paragus first meets Goku during this scene, and immediately identifies him as Bardock’s son due to Goku’s near identical likeness to his father; Goku identifies Paragus as a Sayian, since no human knows or tends to use his real name, Kakarot.
The first time that Broly sees Goku, he immediately recognizes Goku and gets extremely infuriated. Broly and Goku begin powering up in a standoff, until Paragus brings him back under control by using a mind-control device put on Broly when he was a teenager. Unfortunately for Paragus, Broly later finds a way to bypass the mind-control which the device has over him.
During this scene we are told by Paragus that Goku and Broly were born on the same day. Broly was born with an extremely high power level of 10,000 (which is unheard of – most adult Saiyans do not have power levels that high), while Goku had a much weaker 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. During his teen years, Broly’s mental and emotional instability, coupled with his great power, made him highly dangerous. After an incident that cost Paragus his left eye, Broly is fitted with a device meant to keep his power in check. Paragus, who barely survived the ordeal, realized that with Broly’s power under his control, he could rule the universe.
Meanwhile, Broly attacks Goku during the night as a restrained Super Saiyan. Despite Goku’s best efforts, none of his attacks have any effect on Broly, and he is forced to run. 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. At this point, Goku recognizes that the residual power he felt from the Legendary Super Saiyan’s attack is actually Broly’s power – so Broly is the Legendary Super Saiyan – and also realizes Paragus has been lying all along.
Eventually Gohan, Trunks and Krillin bring the slaves they freed back to the palace, and the slaves identify Broly as the one that attacked their planet. Paragus reveals that the new Planet Vegeta is about to be hit by the comet, and that he in fact wants to reestablish Saiyan dominance across the universe using the Earth as their new homeworld. Suddenly, an increasingly agitated Broly goes after Goku shouting, “Kakarot!” repeatedly, transforming into a restrained Super Saiyan while he does so. Vegeta transforms into a Super Saiyan and attacks Broly, who seems oblivious to the attack. Vegeta then hits Broly with his powerful Big Bang Attack, but again, Broly doesn’t even seem to notice. Broly soon rages out of Paragus’ control and, in a very intense transformation, becomes a monstrous behemoth, The Legendary Super Sayian, his true, most powerful form. Vegeta, stunned by Broly’s power, falls to the ground in shock, eyes wide with fear, realizing that Broly is the Legendary Super Saiyan.
Paragus reveals that Vegeta’s father viewed the infant Broly as a threat, and ordered his execution. Paragus attempted to plead with him for his son’s life, stating that the acquired data could be wrong and that Broly could grow up to be a great warrior and an asset to the royal family. King Vegeta stated that, while this was true, Broly could just as easily turn on them, and decided to let father and son die together; Paragus is hit with an energy blast, Broly is stabbed in the abdomen with a dagger, and both are thrown in a ditch. When Frieza destroyed Planet Vegeta, Broly used his powers to surround himself and Paragus with a force field, allowing them to survive the explosion. From then on, Paragus plotted revenge; if not on King Vegeta himself, then his son.
Broly, enraged beyond control, attacks Goku at full force, destroying both the strip mines and the slaves’ home planet in the process. Broly goes after Goku and Gohan in a chase, destroying forests and slaves with a barrage of energy blasts. Broly threatens to destroy the planet if Goku doesn’t fight back, and this triggers Goku to transform to a Super Saiyan. Trunks comes to their aid and both he and Gohan transform too. The battle gets intense with Broly’s strength proving too much for the Z fighters to handle. After Broly threatens Gohan, Goku tells Gohan to run, which he does, but Broly pursues him and grabs his head and throws him into a nearby building, from which he hangs by the cloth around his neck. The Legendary Super Saiyan then takes down both Goku and Trunks. Piccolo shows up after Goku, Gohan, and Trunks have all been beaten, and saves Gohan from a would-be-fatal attack. He then hands each injured fighter a senzu bean. Not long after, the four warriors then decide to gang up on Broly, who has been patiently waiting on a thin cliff. All four of them are pummeled by Broly again. Though Goku and Piccolo manage to land punches on Broly, and Gohan and Trunks’ Masenkos hit Broly directly, none of this has much effect. Piccolo attempts to persuade Vegeta to fight; up to this point Vegeta has been too shocked to do so. While Goku fights ceaslessly, Vegeta rediscovers his inner Saiyan pride, transforms into his Super Saiyan form once again, and joins the fight against Broly. Broly still proves to be too strong, continuing to effortlessly pummel the five fighters. Broly later finds his father, Paragus, attempting to flee inside a Saiyan space pod, planning to leave Broly to die with the destruction of the planet. In retaliation, Broly picks up the pod and crushes it, suffocating Paragus, and throws it directly at the comet, destroying the space pod and Paragus.
Piccolo then tells everyone to give all of the energy they can spare to Goku so he can defeat Broly. One by one they comply, except Vegeta, who refuses to help a lower-ranking Saiyan because of his pride and arrogance. While gathering energy, Goku suffers at the hands of Broly; he is blown across the landscape by energy blasts and very powerful punches, and then repeatedly jumped on, causing him to cough up blood. Finally, on the brink of death, Goku pleads with Vegeta to help him. Vegeta reluctantly does so, and with the combined power of the Z-Warriors, Goku is finally able to defeat Broly with a powerful punch to Broly’s abdomen – right where he was stabbed as a baby during the execution attempt. Everyone then escapes from the planet via the Capsule Corp ship which Piccolo used to reach them. Goku and Gohan teleport home, only to be confronted by an angry Chi-Chi, who is still annoyed at Goku for leaving during the interview. Comically, she passes out when Goku told her “my hobbies are reading and sports”, which was exactly what Chi-Chi told Goku to say in the interview.