Goten and Goku right after fusing.

In the underworld Cell is training to fight Goku once again, yet his efforts are futile. Then Cell realizes that he can just absorb all of the past enemies Goku fought, so he did just that. After he was done absorbing, he escaped Hell and went to find Goku. He found him and they started to fight along with Goten's assistance. Even with both Goku and Goten at Super Saiyan 3 they were both no match to what was now "Ultra Cell". Eventually Goten and Goku fused into Gotenku. They totally dominated the new Cell and obliterated him with a "Super True Kamehameha".