The story starts on Kami's lookout, where anda see Goku and Vegeta talking.
Vegeta: Forget it Kakarot!!!!! Fusion is not happening again!!!! There is no reason for it, so why bother?
Goku: Vegeta, an enemy that we don't even know about could be on their way atau already here that is much stronger than us, so we need to get stronger sejak any means, and training while in fusion will help us.
Vegeta: It was bad enough against Buu, against that ugly freak we fought in hell, and Omega Shenron!!! There is no reason for us to fuse anymore. There is nothing atau no one on this planet who can cause us a threat....