Dragon Ball Z picks up five years after the end of the Dragon Ball anime, with Goku as a young adult and father to his son Gohan. A humanoid alien named Raditz arrives on Earth in a spacecraft and tracks down Goku, revealing to him that he is his long-lost big brother and that they are members of a nearly extinct extraterrestrial race called the Saiyans. The Saiyans had sent Goku to Earth as an infant to conquer the planet for them, but he suffered a severe head injury soon after his arrival and lost all memory of his mission, as well as his blood-thirsty Saiyan nature.
Goku refuses to help Raditz continue the mission, which results in Raditz kidnapping Gohan. Goku decides to team up with Piccolo in order to defeat Raditz and save his son. Goku arrives at the battlefield late, but avenges his fallen friends by defeating Nappa with his new level of power. Vegeta himself enters into the battle with Goku and after numerous clashes Goku manages to defeat him as well, with the help of Gohan and his best friend Krillin.
At Goku’s request, they spare Vegeta’s life and allow him to escape Earth. While Goku recovers from his injuries at the hospital, Gohan, Krillin and Goku’s oldest friend Bulma depart for Namek in order to use these Dragon Balls to revive their dead friends. A fully healed Vegeta arrives on Namek as well which leads to several battles between him and Frieza’s henchmen. Realizing he’s overpowered, Vegeta teams up with Gohan and Krillin to fight the Ginyu Force. After Goku finally arrives on Namek, the epic battle with Frieza himself comes to a close when Goku transforms into a fabled Super Saiyan and defeats him.
Seven years later Goku, and his allies are drawn into a fight against a magical being named Majin Buu. After numerous battles resulting in the destruction and recreation of the Earth, Goku destroys Buu with a “Spirit Bomb” attack containing the energy of everyone on Earth. Goku makes a wish for Majin Buu to be reincarnated as a good person and ten years later, Goku meets Buu’s human reincarnation, Uub. Leaving the match between them unfinished, Goku departs with Uub to train him to become Earth’s new guardian.