Kip. 2: The Enemy Is Goku's Brother?! The Secret of the Mighty Saiyan Warriors!

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The stranger claims that his name is Raditz, and that he is Goku's older brother. Goku cannot remember this, as he had suffered from brain damage caused by a fall when he was a child. Raditz reports that he and Goku are of an alien race called the Saiyans and that he and Goku are among the last four remaining. Raditz then asks Goku to join them in exterminating the population of Earth, taking Gohan hostage when Goku refuses. Piccolo appears and offers to help Goku defeat Raditz; Goku accepts his offer and together they follow after Raditz. Goku and Piccolo confront the Saiyan at his space pod's crash site, but Raditz is not impressed and tells them that the other two remaining Saiyans are even stronger than he is. (Source: Wikipedia)