The Saiyans had sent Goku (originally named 'Kakarrot') to Earth as an infant to conquer the planet for them, but he suffered a head injury soon after his arrival and lost all memory of his mission, as well as his blood-thirsty Saiyan nature.
Five years after the events of Dragon Ball, now a young adult and father to son Gohan, Goku meets his older brother Raditz, who reveals to him that they are members of a nearly extinct extraterrestrial race called the Saiyans.
