Universe 4: The rebirth of Majin Buu is the forty-fifth chapter of Dragon Ball Multiverse and the twentieth special.
In the Year 774 the Saiyans Goku and Vegeta where in the climax of their last stand inside the body of Majin Buu. As Vegeta was about to remove the benevolent Buu from the evil Buu's system, the Majin immediately stopped and absorbed the Saiyan Prince, leaving Goku to fight alone. The warrior used all of his strength to fight but was futile, however Kibitoshin's intervention in the fight outside Buu's body gave Goku enough time to charge a final attack, one that almost manages to destroy Buu, but ultimately fails to do so.
Buu claims victory by absorbing Goku, leaving no one to stop him, but as he was about to destroy Earth he realized that all of his absorption victims where to fond of Earth, thus making him fond of Earth too. Buu ultimately decides to use the Dragon Balls to fix Earth and revive those who perished, casually crossing with the previously defeated Kibitoshin and eating him as a cookie to give him a peaceful death. The Dragon Balls successfully restore the planet and its inhabitants as it used to be, but the revived ones realize that those who where absorbed by Buu didn't came back as they are alive inside of him.
Buu decides to leave the planet and absorbs Bulma before doing so in order to enhance his own intelligence, thus starting his long journey exploring the universe and absorbing anyone he found interesting, occasionally visiting Earth from time to time.