Although Doctor Gero had lost control of Android 18 years ago, she was still his Android. Android even though she betraded the Doctor, married his enemy's best friend Krillin, and had a daughter with him named Maron, was still the doctor's creation. And the doctor still knew all her strengths and weaknesses, because he designed her. Even, a dimension away in hell, Dr. Gero could still access earth through his creation, Android 18.
Ten years later, Android 18 along with the rest of the Z-fighters and their families were attending the World Martial Arts Tournament, some as contenders, some as spectators. This year Android 18 was a spectator, along with her husband, Krillin, and her daughter Maron. Meanwhile, Goku was talking to Vegeta in the line to sign up. Since Android 18 is an Android, she has super hearing.
"Vegeta, there's this guy I really want to fight! He's right over there!" said Goku as he pointed to a dark-skin skinny boy.
"Kakarot, you mean he's the mysterious fighter you want to fight so bad! A kid who doesn't look any older than 10! Really Kakarot! How pathetic!" yelled Vegeta.
"No, when I defeated Majin Buu, i asked to fight him again, as a good person, King Yemma must of heard me. Look at his age, he 10. And look at his name, its Uub. Its Buu spelled backwards. I'm telling you Vegeta that's him. Uub is the final form of Majin Buu reincarnated." said Goku
On the other side of the room, Piccolo and #18 looked at each other and said simultaneously "What!" But Piccolo couldn't help but smirk. "He's been waiting for this fight for 10 years." he said, his devious grin turning into a good-natured smile.
Majin Buu then used his magic to match Goku and Uub together, as Goku requested. Then, Goku and Uub got into the ring together as Android 18, Piccolo, and the others watched. As Uub was getting stronger while fighting Goku, Gero, who was watching through #18, thought that Uub would be a perfect warrior in his vengeance of Goku, first for destroying his Red Ribbon Army, second for destroying his androids. So Android 18 went to the bathroom, and came out Two Androids. Android 18 went back to go sit down. Android 18.5 then went and flew above the ring. Goku then offered to train Uub. So Goku took Uub back to his village on his back, where Goku planned to train him.