Frisk waited until Papyrus and Undyne left the house. It seemed like Sans had never shown Papyrus this part of the house before, so she didn't want to accidentally give his secret away if he was hiding it. She slipped to the back of the workshop and put the key in the door. It opened.
This room seemed very different from every other room in the house. It had tile floors and a green wall. It was like a laboratory of some sort. She knew that Sans was into quantum physics, but never imagined him as one to delve deep into researching it. There were blueprints on the table written with strange symbols she couldn't understand. In the drawer was a badge of some sort. There was a photo of Sans with her, Papyrus, Undyne, Alphys, and Mettaton, which made her smile. He had kept the picture they took together the first night she moved into the skelebro's home right in his secret workshop. Behind that photo was a photo of Sans standing next to three people she didn't recognize. He had never talked about any of these people before. Was he part of a research team? She couldn't picture him in a laboratory conducting experiments. In her mind, she swapped that blue hoodie out for a lab coat on him. It was strange to imagine him wearing anything other than that blue hoodie.
Something else caught her eye. A giant machine covered by a tarp was looming eerily at the end of the workshop. Curious, she walked over to it and lifted the cover. It was a machine of some sorts. It seems like it was used recently, as the dust was empty on the keyboard but present on other parts of the machine that wouldn't usually be touched. She wasn't sure if she should be messing around with the machine, but she reached out for a button anyway.
"I see you've noticed the machine." Sans said out of nowhere. Frisk jumped. She hadn't heard him come in and was startled by his sudden appearance. "Frisk, there's something I wanted to ask you… You're a time traveler, aren't you?" Frisk froze. It's true that she had gone back in time twice, but she didn't consider herself a time traveler. It just so happened that she was able to on two instances. She explained to Sans that she had gone back in time twice, but only once she passed him at the Judgement Hall. For some reason, both times, Sans had known that Frisk had gone back in time and told her two secret code words to give him if she ever went back again, confirming to Sans that Frisk was the one messing with timelines.
"Wait," Sans said. "You've only reset twice?" He was confused. He had been monitoring time for a while and had seen several jumps, not just the times Frisk reset. "I guess I should explain myself."
Sans started from the beginning. He confessed to Frisk that he had lived on the surface many years ago. At the time, he was an aspiring young scientist. Papyrus had not been old enough to remember much yet, but the two of them were living on their own. Sans had been taking care of Papyrus. It was not long before Sans met W.D. Gaster, the royal scientist at the time. Gaster had been impressed with Sans's intelligence and invited him to become part of his official research team. He motioned to the picture, the one next to the picture with Frisk. Aside from Sans, there was another skeleton in the middle of the three lab assistants. This had to be Dr. Gaster. During the course of their experiments, they had figured out a way to disguise themselves as humans and explore the surface. Even Asgore did not know of this. Gaster had been using this research team to carry out observations on the surface. For a time, Sans assimilated himself in a small village on the surface and had befriended a few people on the surface.
"I lived on the surface for a while," Sans said. "Even though I wasn't really human, it was nice to get to meet people. Humans must not be that bad, I thought for a while. At least, I thought that until Asriel came through the barrier carrying a human. It was absolutely brutal." Sans looked down. "The way the humans so viciously attacked Asriel without letting him explain himself, of course, I knew Asriel would never murder his brother in cold blood, but the humans weren't having it. I took Papyrus and hid back underground, afraid that the humans would discover my identity. I went back to Gaster's laboratory to report on what had happened, but when I got there, he and the rest of the lab assistants were gone. The only thing left was a final report from him. He was doing some kind of experiment with determination. I tried to follow up on his research to find out what happened to him, but he was gone. All I found were traces of my old mentor scattered across time and space. I lost my life on the surface and my mentor in the span of a few days." Sans shut his eyes as if he was in pain. This was it, Frisk thought. This is the pain he tries to mask with his humor. It's all out in the open now.
Sans continued. He spoke of how he continued to work in the laboratory and how Alphys had then been appointed as the royal scientist. Alphys had been researching determination while Sans was researching time and space, trying to find a way to piece Gaster back together. Soon after Alphys had done the determination experiments on a flower, Sans's timeline monitor had been going off more than it ever did before. Someone was messing with the timelines.
"That's the machine behind you," Sans pointed to the machine that Frisk had uncovered. "It monitors timelines." He continued to speak of how often the timelines had been resetting and how, because of his research, he could always tell when a reset happened. Every reset put his research toward finding Gaster farther and farther. "It was so frustrating, having to start over and over again. After a while, something clicked in my brain. What's the point? All the work I had been putting in kept going to waste after each reset. I was never going to find Gaster. Nothing was worth it anymore. Frankly, I didn't care anymore. Everything got boring to me. I was swallowed by my existential crisis and my hatred for humanity." Sans stared at the two pictures in his drawer. He picked up the one with Frisk. "Until… you came. Frisk, I've never seen any human like you before. You're the first human to never want to murder any monsters. You've befriended all of us. You… gave me hope again."
Frisk was tearing up. She felt all of Sans' emotions pour into her. His pain, suffering, sadness, and newfound hope. She had no idea that she had really affected him this way. He was even deeper than she ever imagined.
"Sans," Frisk said. "Whatever you're going through, whatever happens, I'll be here for you, ok? You don't have to carry this burden alone."
"It's just really confusing right now," Sans said. "You're not the one messing with the timelines?" Frisk responded that she only jumped back in time twice. Only after her conversation with Sans did she see a glowing yellow light at the end of the hallway. She touched the yellow light and was sent back to before she talked to Sans. In this first time reset, Sans could tell she had gone back, and instructed her to say a secret codeword to him the next time she saw him. Frisk, however, was determined to try and face Asgore and escape through the barrier. What happened next caused her to reset a second time. "Frisk… what happened those two times?"
She took a breath. What happened caused her to come back to the Underground and resign herself to a life away from home. Upon her meeting Asgore, she tried to reason with him, but he wouldn't listen to her. She figured if she weakened him enough, she could try and convince him to find another way to get everyone out. Shy of one blow, she stopped fighting Asgore and tried to talk to him. Before she could, however, a third entity appeared from nowhere and killed Asgore where he stood. Shocked and horrified, the flower she met on her first journey absorbed the six human souls and tried to attack her. She ran away to the glowing light, and reset once again, to before Sans spoke to her. She then spoke to Sans, giving him the secret code word. He then responded that there was another code word to give him if she reset again. She tried again, but it ended up giving the same outcome. It was then that Frisk realized there was no way to escape without killing Asgore, so she turned back and decided to stay in the Underground.
Sans was shocked. She hadn't been the one messing with the timelines. Who was it then? He looked at her again. She was the embodiment of pacifism, even refusing to kill the one thing standing in the way between her and freedom.
"I want to go home," Frisk said. "But I'm not killing anyone for it. There has to be another way. Sans smiled and brushed the hair out of her face.
"I'll help you," Sans said. "We can do it together."
The feeling of connecting to Sans's soul filled Frisk with determination.