It had been a half a year since the human had saved the monsters of the Underground. Once aboveground, with Frisk as the ambassador of the monsters, they had adjusted very well. As it had turned out, they had been underground for so long that a good majority of humans outside of the mountain were unaware that they had even existed—monsters had simply became myth and legend rather than fact. So the human population's reaction to their appearance was more of shock than fear. And yeah, there was a fair share of prejudices among the humans, but that was to be expected. But no one was being killed—and that was more than Sans had expected.
At any rate, it had been, by all accounts, an average day. Frisk and Toriel had just left from a visit to Sans and Papryus's house. Papryus was now busy doing all the dishes from the spaghetti dinner he had made them, and Sans was at the kitchen table, doing a crossword puzzle.
"SO BROTHER, WHAT DID YOU THINK OF FRISK? THEY SEEMED TO BE DOING WELL. MORE IMPORTANTLY, WHAT DID YOU THINK OF MY SPAGHETTI?!"
Silence.
"BROTHER?"
Papryus turned around. Sans was no longer doing his crossword. Instead, he was staring straight ahead, and his eye sockets were empty. The permanent grin was still there, but Papryus knew he wasn't smiling.
"BROTHER, ARE YOU ALRIGHT?! WHAT IS GOING ON?!"
Sans blinked slowly. "hey bro."
"WHAT JUST HAPPENED?! I DEMAND AN ANSWER! ARE YOU OKAY?!"
"uhhh yeah i'm fine bro. i'm just... just really tired all of a sudden. i think i'm gonna go take a nap or something."
Papyrus jumped up and down in frustration. "I KNOW THAT'S NOT IT! THERE'S SOMETHING YOU'RE NOT TELLING ME! I DEMAND TO KNOW!"
"hey i'm sorry bro, maybe i'll tell you later. i'm not sure what's going on right now." Ignoring Papryus's further protests, he shuffled into his room and shut the door. And locked it.
Truth was, he had just felt something. A shift. A shift... an anomaly in the timeline. Not a lot shocked Sans—after all, his knowledge far surpassed Papryus's or even Alphys's. But this...
He hadn't felt anything like this since Frisk had walked out of the Ruins.
And furthermore, there was something... different about this. After making sure the door was locked, Sans checked his machine, the one that detected shifts like this. The one that he and Gaster had built, years ago.
Sans's intuition had been right—there was a change detected by the machine. But it wasn't anything he had seen before. Perhaps it could be an interdimensional type shift, as the machine didn't detect any timeline distortions (as it had when Frisk had arrived). At this discovery, Sans breathed a sigh of relief. He didn't need to relive that again.
But this whole thing was puzzling and intriguing. He considered presenting Alphyus with this development, but for the time being, decided to keep it to himself.
All he knew was, something was coming. And he had to be prepared for it.