Goku, the hero who destroyed the evil of Frieza, Cell, and Buu in DBZ, learns that an old foe, Emperor Pilaf from DB has captured the 7 magical Black Star Dragon Balls, which, at the cost of the planet the balls are on, can grant any wish. Pilaf wishes that Goku is a child, and Goku becomes a child. In order to save both the planet and restore Goku to adulthood, Goku must allience himself with Trunks and his grandaughter Pan to find all 7 Balls all over the universe. Over time, they encounter foes like Baby, Super 17, and the invinsible Shadow Dragons.
Taking place 10 years after the final Dragonball Z story, Pilaf finally manages to get all 7 dragonballs and makes a wish. Unfortunately for Pilaf, he gets flustered by the presence of Goku and wishes for Goku to be a child again so that Pilaf could "teach him a lesson." Pilaf gets his wish, and Goku is off on an interstellar trip to gather the 7 "Black Star Dragonballs" that can reverse the wish and return him to adulthood.

Source: Anime News Network

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Dragon Ball
Dragon Ball Z
Dragon Ball Kai