Dragon Ball Z takes place five years after the events of the Dragon Ball series and follows the adventures of a now young adult Son Goku, who is also a father to his son Gohan. Their peaceful life is disrupted when a humanoid alien named Raditz arrives on Earth in a spacecraft. To Goku's shock, Raditz reveals himself to be Goku’s long-lost older brother and a member of the endangered elite warrior race known as the Saiyans (サイヤ人, Saiya-jin).
Goku, whose original name is Kakarot (カカロット, Kakarotto), was sent to Earth to escape the destruction of their home planet, Vegeta. Raditz explains that he, along with two more formidable Saiyans, Vegeta and Nappa, are the last survivors of their race following the annihilation of Planet Vegeta. Attempting to recruit Goku into aiding them in conquering worlds, Raditz's offer is met with refusal. In retaliation, he swiftly takes both Goku and Krillin down with a single blow, abducts Gohan, and threatens Goku with his son's life unless he kills 100 humans in the next 24 hours.
With time against him, Goku forms an unexpected alliance with his rival, Piccolo, who is also a target of Raditz's earlier onslaught, to rescue Gohan and confront Raditz. During the intense battle, Gohan's latent rage surges, granting him a momentary increase in strength that surprises both Goku and Piccolo as he fiercely protects his father. The confrontation reaches its climax when Goku manages to hold Raditz in place, allowing Piccolo to unleash a devastating attack known as the Special Beam Cannon (魔貫光殺砲, Makankōsappō; lit. 'Demon Penetrating, Killing Ray Gun'), mortally injuring them both. In his dying breaths, Raditz warns Piccolo that the two other Saiyans are more powerful and will come for the Dragon Balls in one year.
Recognizing Gohan’s potential, Piccolo takes on the task of training him in isolation to prepare for the looming threat of the Saiyans. Meanwhile, Goku embarks on a perilous journey through the Afterlife along the million-kilometer Snake Way to train under King Kai, who imparts crucial techniques, such as the Kaio-ken (界王拳, Kaiōken; lit. 'Fist of Kings of Worlds') and the Spirit Bomb (元気玉, Genki Dama; lit. 'Energy Sphere').
Despite their antagonistic beginnings, Piccolo develops a bond with Gohan as he mentors him in survival and combat skills. As a year passes, Goku is revived using the Dragon Balls, but he learns that he will be delayed until after the Saiyans arrive due to having to traverse Snake Way again. His friends, realizing the urgency, attempt to fend off the Saiyans but fall to Nappa and Vegeta, the latter known as the "Prince of All Saiyans." Tragically, the battle leads to the deaths of several allies, including Yamcha, Tien Shinhan, Chiaotzu, and Piccolo, who is greeted with the grim fate of erasing both himself and the Dragon Balls from existence.
Goku finally reaches the battlefield, fueled by vengeance and urgency. He quickly defeats Nappa with ease, causing Vegeta to execute Nappa for his failures. Goku immerses himself in a fierce struggle against Vegeta, employing various levels of the Kaio-ken technique, although it takes a toll on his body. Vegeta, using an artificial moon, transforms into a colossal Great Ape, overwhelming Goku. However, Krillin and Gohan, sensing danger, return to aid Goku and, with crucial support from Yajirobe, manage to cut off Vegeta’s tail, returning him to his normal state. Goku gives Krillin a Spirit Bomb, which results in significant damage to Vegeta, ultimately leading to his defeat when Gohan transforms into a Great Ape and crushes Vegeta, forcing him to escape, vowing vengeance against Goku.
During the chaos, Krillin overhears Vegeta mention the original Dragon Balls from their alien homeworld, Namek (ナメック星, Namekku-sei). While Goku recuperates from his injuries, Gohan, Krillin, and Bulma set off for Namek to revive their lost friends through the Dragon Balls. Upon arrival, they discover that Vegeta and the powerful Frieza are already hunting the Dragon Balls for immortality, prompting a high-stakes game of strategy and conflict.
Vegeta’s newfound strength, cultivated after coming back from near-death experiences, sees him increasingly rebelling against Frieza. The situation intensifies as they all vie for the Dragon Balls, pitting Frieza and his mercenaries, the Ginyu Force, against Goku, who eventually arrives just in time to save Vegeta, Gohan, and Krillin from certain doom. A legendary clash ensues, leading Goku to heal and uplift his allies just before facing off against the tyrant himself.
Goku learns about Frieza’s history of destruction against the Saiyans and soon enters a climactic battle. Although Goku’s resolve is unwavering, he finds himself outmatched, leading him to request assistance from the Spirit Bomb. In a moment of rage and desperation, after witnessing Krillin's death and Frieza’s relentless cruelty, Goku transforms into a Super Saiyan, marking an iconic moment in the saga. With his newfound power, Goku engages Frieza, ultimately surmounting the overwhelming odds and escaping as Namek meets its fiery demise.
Following a tumultuous period resulting from these confrontations, Goku's friends anxiously await his return when a formidable foe, Garlic Jr., escapes his imprisonment seeking vengeance against Gohan. The heroes engage in combat against Garlic Jr.'s ever-growing power, ultimately finding victory when Gohan reaches the brink of his latent potential, sending Garlic Jr. back to the depths he came from.
One year later, Frieza makes a shocking reappearance on Earth, but is swiftly dealt with by the mysterious Trunks, a young warrior claiming to be the son of Vegeta and Bulma. He warns Goku about Androids created by Dr. Gero that threaten his friends' lives. As the android saga unfolds, friendships are tested, new alliances are forged, and Goku learns to harness the power of the Super Saiyan form once again.
As time progresses, the saga introduces new challenges that include the vengeful androids, the sinister Cell, and the existential threat posed by Majin Buu. Through decisive battles, sacrifices, and transformative moments of growth, Goku and his friends strive to protect Earth from annihilation, witnessing the birth of alliances and the deepening of existing ones. The epic narrative culminates with Goku's journey featuring the restoration of peace, the resurrection of his friends, and ultimately setting the stage for future generations to carry on the legacy of protection and bravery.
