You were fighting Majin-Buu after Vegeta sacrificed himself to defeat Majin-Buu by using Final Explosion (self destructing), which as you know, didn't work. So to buy time for Piccolo, Trunks, and Goten, you decide to fight Majin-Buu to buy time while they learn some stuff in the Hyperbolic Time Chamber. You turn SSJ2 to hold Majin-Buu off and fight, but . . . it didn't end well, you were injured, and tired, as you fought all the way to the Hyperbolic Time Chamber. You arrive with Majin-Buu still fighting until you and Majin-Buu sense an energy. Since you and him were flying, you look down and see Goten and Trunks but . .. they look different . . . they fused, now, they are Gotenks. You Instant-Transmission next to them and sit on the floor as you see the kid fusion look at you with a determined smile to beat Majin-Buu.

G: "Well, I guess you need a rest. Don't worry, I'll handle him." He looks up at Majin-Buu.