The story starts on Kami's lookout, where you 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 or already here that is much stronger than us, so we need to get stronger by 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 or no one on this planet who can cause us a threat. Both of us at Super Saiyan 4 is more than anyone can handle!!!
Little did Vegeta know that down in hell, Dr. Gero was hard at work with a new project.
Dr. Gero: Yes!!!! they are finally complete!!!! now time to open their doors and let them come out!!!
Gero walks to the wall, presses the button, and the two doors open up. Out comes Goku and Vegeta.