The day everything went wrong...
After what seemed like forever, yet to most of them, was just 3 days, They were on the surface. Granted sans knew this wasn't the first time. He just hoped it would be the last time Frisk reset. They decided to stay at a hotel where they were given very weird looks from the other guests. Frisk took the liberty of booking the rooms seeing as the humans had yet to like the idea of monsters on the surface. The room pairings went as such, Undyne and Alphys shared a room with the amalgamates to make sure they were given food on time, Sans and Papyrus were roomed together of course, Flowey Asgore Toriel and frisk all shared a room mostly so Frisk could get to know goat dad and Toriel could keep an eye on him and Flowey. Oh yea! Napstablook and Mettaton were rooming together to reform their bonds. It was a nice hotel and everyone was having a great time! Well except Flowey... He didn't seem to like it much.
As time went on it seemed like life couldn't be better for everyone. They soon had bought a large house big enough to house all of the monsters from the underground. Everyone could have had their own rooms of they wanted! Frisk's room had mostly pictures of her and her friends, and movie posters from Mettaton's past performances. She liked how simple her room was, Papyrus was starting to get much better at his cooking, Undyne didn't throw things at everyone as often... Mettaton, Napstablook, and Shyren were stars, Alphys had started up her own anime for the Internet, Sans decided to become a scientist again, Toriel and Asgore were getting along for the most part. Yep it seemed like nothing could go wrong! At all...
Well it seemed like that way tell on a normal sunny day... Frisk didn't come home... They weren't to worried at first, after all she did wonder off and explore a lot. But when she still hadn't come back near dinner time they realized something might be wrong... She always came home before dinner, no matter what. So Sans, Jerry, Snowdrake, and the dog amalgamate went out to look for her. They thought maybe she just got lost or forgot what time it was, it wasn't to often but he could happen right? They searched down town thinking maybe she was there. Sadly they found her... The dog thing started howling near where she was which was close to any allyway near a busy street, when sans and the others went to see what it was howling at... It was horrible, she was all red and clearly wasn't going to just wake up... Sans was desperately shaking Frisk telling her she needed to get up, that everyone was worried, that if this was some sick joke he wasn't laughing... Her eyes wouldn't open. Snowdrake called everyone that was waiting at the house to inform them of what happened.
Everyone was devastated, the girl who had brought them together and set them free was killed so easily... From what coroners told them she had tried so very hard to keep going... She had died by an accident in the streets and crawled away from it and ended up spending her last moments alone... Everyone could easily imagine her calling for help but her voice being drowned out by the honking cars and busy city sounds. Everyone was crying but Sans. He was glaring at the ground, shouldn't she have reset already? Shouldn't she have not died but reset? Why hadn't she just reset? He made her promise not to reset unless something happened to one of them like Undyne getting arrested again... But she wasn't one of them, she was just a human that wanted to stay with them... He forgot she wasn't 'one of them' of course she wouldn't reset if something happened to herself! He wanted her to come back he didn't care if he had to live twenty resets he just wanted her alive again.
He spent the rest of that horrible week locked in his room trying to come up with a way to fix this... "Flowey lost the ability after the barrier fell... Maybe... Maybe I can make something to fix this... A machine to turn back time?" He knew messing with the code was a bad idea... But he didn't care. He wanted her back. So he spent a whole month working on a new machine, once it was finished he doubted if it would really work... He turned it one anyways...