Dragon Ball Kai The Lost Episodes

A Secret Plan to Defeat Buu! Its Name is Fusion (6x7)

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Bulma's group goes to seek out the Dragon Balls so they can revive all the spectators that Majin Vegeta killed. They come up against a large dragon while trying to get the last Dragon Ball. Piccolo and Krillin leave Goten and Trunks in the care of Dende and Mr. Popo. They are soon joined by Goku, who learns about the situation. Goku states he is not strong enough to defeat Majin Buu. He mentions having learned a technique known as Fusion, which allows two similarly built fighters to merge to become a single powerful fighter. Goku had originally planned to do this himself with either Gohan or Vegeta. Goku realizes from Mr. Popo that he can instead teach this technique to Goten and Trunks due to their similar size.