Long ago, on the planet Vegeta, the intergalactic tyrant King Cold visited the Saiyans with an important announcement: he would be stepping down from his throne and passing his responsibilities to his son, Frieza. As a show of "good will", Frieza bestowed the Saiyans with new tools called "Scouters", eye-mounted monitors that could detect others' battle power levels and their locations, a function he showcases by killing the assassins targeting him.
King Vegeta is shocked at Frieza's brutality, but is nonetheless determined to one day shake off the yoke of Frieza's oppression, an ambition he would bestow upon his son, Prince Vegeta IV. While admiring his son's latent battle power one day, however, the king notices another child in the royal incubation room; Broly, the son of a low-level general named Paragus. According to the doctors, Broly was placed in the same room as the infant prince because his power levels, though wildly fluctuating, could exceed even the prince's: he could even be the legendary Super Saiyan!
Unable to tolerate having a commoner possibly surpass his son, King Vegeta has the infant Broly exiled to the distant and barely-hospitable planetoid of Vampa. When Paragus learns of this, he steals a spacecraft and follows Broly to Vampa, vowing to raise him by himself and train him so that he may one day exact revenge against the cruel king...
Five years later, Frieza recalls all of the Saiyans throughout the galaxy to the homeworld. The Saiyans are excited over what is to come, possibly a new conquest and a chance to fight. One Saiyan, however, believes something to be horribly amiss: Bardock, a low-level soldier. When he returns to Planet Vegeta, he reunites with his wife Gine and asks about their children. Their elder son, Raditz, was off-world on assignment with Prince Vegeta. Their younger son, Kakarot, is almost ready to emerge from his nursing pod. Worried for his newborn son's safety, Bardock has Gine accompany him as he places young Kakarot into a space pod set to travel to a backwaters planet called Earth: with his power level still low, Kakarot would avoid detection by Frieza and be able to survive, Bardock intending to retrieve him and bring him back home if Bardock's suspicions turn out to be for naught.
Tragically, Bardock's concerns were warranted: concerned that the Saiyans would revolt one day, Frieza decided to head off any rebellion by them and recall them all to Planet Vegeta, so that he could exterminate the entire race in one fell swoop. In spite of Bardock's fiercest resistance, Planet Vegeta was destroyed, along with the near entirety of the Saiyans.
Decades pass, and Kakarot, now rechristened Son Goku, has since grown into one of the most powerful warriors in all of Universe 7. After competing in the Tournament of Power, Goku has redoubled his efforts in training so that he can stand toe-to-toe with the rivals he met from across the multiverse, with Vegeta training alongside him so he wouldn't fall behind. Thankfully, their mutual friend and Vegeta's wife, Bulma, had created a special island resort where they could spar to their heart's content.
Trouble is brewing over the horizon, however, as Trunks calls Bulma to inform her that someone had absconded with the Dragon Balls that she had collected to wish upon Shenron to reverse her aging by five years. Security footage reveals the burglars to be wearing armor indicating they were with Frieza's army, most likely low-level grunts that could evade Goku and Vegeta's detection. There was still one Dragon Ball left in the Ice Continent, so Goku, Vegeta, and Frieza make their way to head off Frieza's forces.
Meanwhile, as Frieza revels in his plot to wish for Shenron to increase his height by five centimeters (his time in hell made eternal life less than appealing, and he was worried his men didn't take him seriously), two grunts, Cheelai and Lemo, pick up an old distress signal from a nearby planetoid. When they go to investigate, they find two Saiyans: an adult Broly and an elderly Paragus, both of whom had no idea what fate befell the Saiyans. Cheelai and Lemo bring the two to Frieza, who explains that Planet Vegeta was destroyed by a "stray meteorite", taking the Saiyan race with it. Nonetheless, Prince Vegeta still lived. Paragus vows to aid Frieza in his endeavors, in exchange for leaving Prince Vegeta alone, so Broly could kill him.
Broly becomes friends with Cheelai and Lemo, telling them about his growing up on Vampa. During a meal in the mess hall, when another soldier taunts his new friends, Broly attacks him, only to be subdued by the shock collar Paragus forced him to wear to control his otherwise berserk temper. Shocked at Paragus's cruelty towards his own son, Cheelai stealthily takes the switch for the collar from him and destroys it.
Eventually, Frieza's ship arrives on Earth, with Frieza, Paragus, and Broly emerging to confront Goku and Vegeta. Paragus sics Broly on Vegeta, and the two engage in a fierce battle. At first, it seems that Vegeta is a match for berserk Saiyan. Over time, however, Broly adapts to Vegeta's fighting style, such that he can keep up with the Saiyan prince even as he starts tapping into the power of the Super Saiyan God! With Broly's rage becoming increasingly fierce, Paragus reaches for his shock collar switch, only to find it gone: there was nothing to control Broly now!
Goku intercedes for Vegeta and fights Broly, himself, but Broly continues to adapt and get stronger, eventually overwhelming Goku. With some psychic inspiration from Piccolo, Goku evens the playing field again by going Super Saiyan Blue, and seems to have Broly on the ropes. It is then, however, that Frieza recalls how Goku became a Super Saiyan after watching him murder Krillin. With that in mind, Frieza murders Paragus and calls out to Broly, telling him that his father died.
The sight of his dead father causes Broly's rage to boil over, and he transforms into a Super Saiyan, one strong enough to brutalize even a Super Saiyan Blue! Not even Vegeta going Super Saiyan Blue can make a difference. Desperate and running out of options, the two take advantage of Broly's blind rage by tricking him into fighting Frieza while they use Instant Transmission to meet up with Piccolo. The only hope the two have left is in performing a Fusion, the same way Goten and Trunks did while fighting Majin Buu. Vegeta is less than enthused with the idea, but nonetheless agrees to fuse with Goku.
Frieza fights Broly, attaining his Golden Form in a vain attempt to keep up while Goku and Vegeta attempt a Fusion. Although it takes them a few tries, eventually the two successfully fuse together into a new warrior: Gogeta! The fused Saiyan warrior returns to the battle and engages Broly once more. This time, the power of two Super Saiyan Blues fused together proves to be just the thing that can finally defeat Broly.
Cheelai and Lemo, however, are afraid the battle would end in the death of their friend. In a desperate attempt to save Broly, they gather the Dragon Balls, summon Shenron, and make their wish: to return Broly to Vampa.
The battle is over, and the day is saved. Cheelai and Lemo make their getaway, now wanted by both the Galactic Patrol for their previous crimes and the Frieza Force for their betrayal. Frieza concedes to Gogeta and takes his leave, but he will not give up on his ambitions or his vengeance so easily; he will return to Earth one day.
Three days later, Cheelai, Lemo, and Broly reunite on Vampa. As the three are left wondering how they will survive, Goku suddenly appears via Instant Transmission. While Cheelai and Lemo are suspicious of him, Goku shows that he means no harm by bringing gifts for them; a Capsule Corp house to live in, plentiful food, and two Senzu Beans for emergencies. When asked why he was being so kind, Goku explains that what he really wants is the chance to fight with Broly again, a prospect that makes Broly happy, now that he has found another new friend!
Before taking his leave, Goku introduces himself formally to them, but says that Broly can call him "Kakarot".
