A few weeks after Frisk's birthday, Frisk was asleep and they woke up to some rustling downstairs. Frisk open their door to see Sans leaving the house. Sans always left the house like this for quite some time now and usually Frisk chose to ignore, but Frisk curiosity overcame them. They ran downstairs towards the front door. They slipped their boots on that were near the door and bolted after Sans.
Frisk stayed a good 30 feet away from him. They knew if they walked up to him he would tell them to go home. Sans turned around a few times, like he could sense Frisk, but frisk would hide behind a pole or bush. Frisk didn't like following Sans, but he always looked tired and has been going out a lot. Sans was walking straight to the underground.
He entered where the barrier used to be and sighed. Frisk was confused why Sans was coming back to this place, and they wondered if this is where he kept going. He led them straight to the CORE. He went into a room and Frisk heard him yell, "Y-you're here! It worked!" Frisk heard a strange voice they never heard before, "yes my child. But I cannot stay for long." Frisk took a step closer, but the board they stepped on creaked. The figure took notice to them and came towards them. He moved like a shadow. He scooped Frisk up in both arms and asked, "What's this?" The figure looked at Frisk unsure what to do with them. Then blue mist coiled around Frisk and they were ripped out of the man's arm into Sans.
Sans eye was glowing, and Frisk felt a surge of fear to guilt. "Kid!? Why are you here!" Sans asked his eye dimming, yet he still looked disappointed and mad. Frisk looked up at the figure. "hello child," the man said. He looked badly injured with cracks in his skull and he looked lost. "s-sorry I uh," Sans tried to apologize, but the man stopped him, "My child, it is fine. If I'm not mistaken this child is yours?" Sans blushed and stammered, "Uh... I am married to their mom, so I guess that m-makes me their dad, but they never call me that." Frisk whimpered and looked down.
"Child it would be rude to not introduce myself. I am Gaster," the man who is Gaster stated. "hey kid I don't want you to walk home by yourself (also your mom would kill me) can you wait outs-" Gaster held up a hand, "no. no. let the child stay." Sans looked surprised but nodded. They talked for a while and Frisk sat against a wall trying not to fall asleep. Gaster explained that he had been watching over the family. "I'm happy Frisk broke the barrier, but the child is not as innocent as everyone thinks." Frisk looked confused. "They have… Done things in early timelines." Frisk never really remembered true resets, unless they come in nightmares or Chara tells them in their dreams. "it does not matter," Gaster said coldly, "they have corrected themselves. No resets?" Frisk cringed at the thought and shook their head. Gaster's image started fading.
"I'm… I'm sorry I cannot stay. I must go now. Do not worry though," Gaster said hugging Sans, "I'll watch over you. I may come back one day! I love you Sans. I love you and your brother a lot." Gaster's image seemed to fade and shatter away. "goodbye father." Sans said, a tear dripping down his cheek.
Sans took Frisk's hand and they started walking home. Sans had started crying and tried to hide it. "Hey kid, don't look to sad for me! You got to meet my dad. Uh… your… technically speaking your grand- I'm making this awkward aren't I?" he looked down sheepishly. Frisk gave him a hug and put their finger to their lips. Sans nodded and looked ahead. For a few minute they walked in silence until Sans broke it. "Kid why did ya follow me?" Frisk shrugged. Sans sighed and squeezed their hand.
When they got home, Frisk was about to face plant on the ground and sleep. Sans helped them take off their boots and carried them to their room. Sans tucked them in, but Frisk didn't want Sans to leave. "Yea Frisk? Whatcha need?" Sans yawned. Frisk didn't like Sans being upset, so they patted the empty spot next to them to signal him to lay down with them. "kid," Sans grin got a bit bigger. Frisk clasped their hands together and gave puppy eyes. Sans shook his head and laid down next to Frisk. "alright kid, you win. Why do ya want me to lay here?" Frisk told sans, "Because I'm afraid of a nightmare." He nodded and got himself comfortable. Frisk fell asleep easily knowing Sans was next to them. Their dream wasn't so relaxing.
Chara was in it and she spoke angrily, "That comedian. He… Why do you get close to him? You should know this. He can easily destroy you. His father… W.D. Gaster. He used to be the royal scientist, real genius too. Started the DETERMINATION experiment. His son, Sans, caused Flowey a lot of resets ya know? That comedian should have been easy. Don't you remember? Ha. Of course not. He had only 1 hp and did 1 damage. Well I helped you. He should have been the easiest, but was stronger than I anticipated. Oh? Are you trying to wake up? Go ahead! You're going to have my words in your head. Goodbye now. Haha." Frisk woke up in a cold sweat. Sans wasn't lying next to them anymore and it was already 6:16 am. They had to get ready for school. They got dressed and brushed their hair. They watered Flowey who kept yelling. They walked downstairs and surprisingly Sans was at the table.
Frisk sat next to Sans, while Toriel served eggs and ham. After eating frisk brushed their teeth and Toriel drove them to school. Toriel teaches Frisk's 5th grade class, but next year Frisk will be in middle school and Toriel will start teaching 4th grade. What Toriel was teaching was just review, but she tried to make it as fun as possible. The kids had gotten used to a monster teaching the class and now they all love Toriel.
At the end of the day Frisk had gotten an invite to a birthday party. They stuffed the card into their pocket and walked out to the walkers' area. Toriel stays after school to grade papers and do teacher stuff, so Sans walks Frisk home almost every day. "heya kiddo," Sans smile got bigger. Blue mist coiled around Frisk's Backpack into Sans hands. He pulled out Frisk homework folder and looked through it. "Frisk you weren't tired in class were you?" Sans asked putting their homework folder back. Frisk shook their head. They fished the invitation out of their pocket and handed it to Sans. He read it and smiled. "your mom is gonna make you go, ya know." Frisk nodded and laughed. They finally got home.