Dragon Ball Z, Vol. 24 by Akira Toriyama

Breaking from his master Bobbidi, the rampaging Boo turns the entire world into his candy store. To fight back, Goku transforms into his most powerful form yet, but the world's only hope may lie in Trunks and Goten, the world's youngest and most promising fighters. Using an alien fusion technique, they attempt to merge together to form a single, even more powerful hero. Meanwhile, as Boo destroys city after city, the people of Earth call forth their secret weapon the seemingly invincible (but actually incompetent) Hercule.