Gaster opened his eyes, expecting to see sans and Alphys, but instead seeing the king's two kids before they had died. Then, he saw himself and the king talking. I must be in a dream... or a memory. He thought. "The middle of the Core is going to explode, your majesty. I need to go in and save it." Gaster heard himself say to the king.
"You're my best scientist! I can't have you risking your life to save everyone else. Besides, I need you to come up with a vaccine. My child... they're sick, and Asriel will not forgive me if I let them die." Asgore said, but Gaster could see the greed in his face He was confused as to why he didn't notice it back then. But right at that moment, the king's child collapsed.
The king rushed Gaster into the room where the sick child was sleeping. "Gaster... you have to make a vaccine" The king was about to break down.
"I-I'm sorry,your majesty... but I-I have to go save the Core..." Gaster ran out of the room, and ran all the way to the Core's center building. "I have to do this..." Gaster said to himself. 'Why am I doing this? I don't remember a thing about THIS...' Gaster watched himself jump into the center, his eyes turning blue and a protective fireball forming around him. Gaster watched his soul burn up in the Core... but the Core stopped. Everything stopped. Gaster stopped falling, and everything was still. What did I do?
Then, Gaster woke up.