The Enemy is Goku's Brother?! The Secret of the Mighty Saiyan Warriors! (敵は悟空の兄!? 最強戦士サイヤ人の秘密, Teki wa Gokū no Ani!? Saikyō Senshi Saiyajin no Himitsu) is the second overall episode in the Dragon Ball Z Kai series. The episode first aired on April 12, 2009. Its original American air date was May 24, 2010. Its Dragon Ball Z counterparts are "Reunions" and "Unlikely Alliance".
The stranger reveals that his name is Raditz and that he is Goku's older brother. Goku cannot remember this, as he suffers 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. He then asks Goku to join them in exterminating the population of Earth, taking Gohan hostage when Goku refuses.
Piccolo appears and offers to help him 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 there are two other Saiyans that are even stronger than him.
- During Raditz's explanation about the Saiyans, Vegeta is seen with his original incorrect palette. It is interesting to note that his color scheme is "corrected" a few episodes later.
- In original dubbing, Raditz says "What the hell happened to you?" about Goku's memory. In Nicktoons edit, he says "What has happened to you?"
- This episode begins with a flashback of Son Goku and Piccolo’s battle during the 23rd World Martial Arts Tournament, which was not originally shown in Dragon Ball Z.
- When Goku is attacked By Raditz using Heavy Finish, his cries of pain as well as his hand gestures are similar to how he felt when he had a heart virus, as well as Future Goku later on in the Androids saga.