After the RESET, Frisk decided to take the pacifist route. When Frisk was visiting Waterfall, Frisk discovered an unknown room, where there stood an unidentified creature, Dr. Gaster.
Sans wanted to talk to Frisk about the reset at Grillby's. However, he quickly noticed that Frisk was missing.
Gaster wanted to capture Frisk's soul, since he discovered that Frisk's Determination and SOUL was enough to bring him back to the real world.
After Frisk was trapped in the room, Gaster began to attack them several times. However, Sans came in and tried to protect Frisk, but he wasn't powerful enough to stop him. Then, Sans got the idea of absorbing Frisk's soul in order to defeat Gaster. Luckily, Sans and Frisk won the battle. Sans confronted Gaster and told him that killing others isn't the correct answer to get out of the Void. Gaster refuses to listen and attempts to attack Sans from behind, but he is impaled by bones before the attack hits Sans. Gaster turns to Dust and Sans resets the timeline again.