I can't explain GT without laughing, so I'll do it quickly according to points:
1. I don't remember who uttered it, but the dragon overheard something about Goku being a kid again and granted the wish - thus turning him into a kid.
2. Goku can only resume his adult form via SSJ4, which according to...I think it was Toriyama, was created for no other reason than "The enemies were getting stronger". SSJ4 is, as far as GT is concerned, that of "pure" form and the ascension to this level requires looking at a full moon.
3. Vegeta is possessed by a Tuffle (Bebi) and eventually becomes Super Bebi...or whatever he's called here.
4. There are two Android 17's (one from the past and the other from the future), and they fuse together to make Super 17 - whose defeated by the Dragon Fist.
5. Black Star Dragonballs turn into a nightmare of seven dragons each with their own unique ability.
6. Li Shenron (Omega Shenron in the States), the 1-Star Dragon, henceforth the strongest of the seven starts kicking Goku's ass, Vegeta pops up, ascends to SSJ4 with the help of Bulma...the two of them fuse (Gogeta), and begin toying around with Li but ultimately falls short because despite the original limit being that of 30 minutes, it was effectively halved because of the immense power.
7. Goku defeats Li with a Spirit Bomb, fuses with dragon, leaves Earth in Vegeta's hand.
8. Eleventy years later we see descendants of both Goku and Vegeta participating in a martial arts tournament and that's where it ends.
I left out a good chunk of details because I either can't remember it precisely, didn't care to mention it, or flat out forgot.
I hate GT. It needs to burn in a fire.
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