Dragon Ball Kai The Lost Episodes

Birth of a Merged Superwarrior His Name is Gotenks! (6x16)

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After some dubious persuasion from Goku, Old Kai reveals he has the ability to draw out someone's true power beyond their limits. This turns out to be a lengthy process that can take over a day to complete. Meanwhile, Goten and Trunks try out the Fusion Pose for real, but an error in the Fusion Pose results in the merged fighter, Gotenks, ending up with a failed transformation. After another failure, Goten and Trunks eventually perform a successful fusion and transform into the true Gotenks. Gotenks is confident that he can defeat Majin Buu with his current power level. He goes off in search of Majin Buu despite Piccolo's warnings.