Essentially [Y/N] and Frisk trade places once Flowey forces Frisk to reset with the souls. That is all.

You'd heard the stories, but you never thought you'd be sent back in time to time of the war. Frisk's story rang clear in your mind. Their story about how they fell into an hole and were trying to save their monster friends. You'd been skeptical at first, now you knew. You tried to recall any names they'd said. "Uh.. uh... S....Sam? No, no, it was unique.. uh.. Sa... Sa... San? Sans! One of them was Sans!" You wanted to get back to your time. You wanted to find Frisk, apologize immensely, and help them free the monsters. First though, you had to find Sans.

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Being a child, it was actually rather easy to hide in monster territory. You weren't to eager on getting caught. You'd never met monsters like Frisk had. You had no clue how to interact with them. It was the yellow jacket that caught your attention. He was a skeleton that looked about your age, yellow jacket, blue goggles, black and white shorts. A much younger skeleton held onto his tail. Was that Sans? He looked like the person in Frisk's picture.. kinda. You wanted to call out but the skeletons weren't the only ones around. Two goats, a fish lady, a lizard, and some gorgon creature was playing with them. If you didn't know any better, you'd say they were all Frisk's friends but.. right. Went back in time. Of course they were Frisk's friends but younger.

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You watched, to nervous to join. What if you insulted them somehow? What it their parents were watching and tried to kill you? Lucky for you, Sans raised his nose to the air, head twitching in your direction. "let's play hide and seek."
"Heck yeah! I'll beat all you losers!" The fish ran off. "U-undyne wait!" The dino chased after 'Undyne'. "I'll take Papyrus and hide." Sans grinned and led the smaller skeleton to the female goat. She took Papyrus' hand. "Let's go, Pap."
"OK!" Sans watched them go. "thanks tori." The gorgon thing was following Asgore around like a lost puppy. (Was it Asgore? Undertale Wiki says so, but I know wiki can be wrong sometimes..) Finally Sans was alone. "you can come out now, human."

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You nervously came out of hiding. "Uh.. S-sans..?" He perked. "how..?" He looked away. "i knew something was wrong. you know my name for a reason, don't you?" You nodded. "Um.. I was sent back in time.. and I need help.." He turned back to you, golden slits scanning your features. "back in time.." A gold blush crept into his cheekbones. "Sans?" He shook his head. "i believe ya. something has felt.. off for awhile now. if the timeline somehow went back.. not for you, but for all of us.. then it makes sense. we just have to figure out how."
"Frisk said something about using souls to leave.. think if we find these souls..?"
"might just work.." He agreed. "stay here, i'm gonna go find the others before we go."

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After he dropped Papyrus off at home, we started looking for the souls. "what do they even look like? i mean, i know their human souls, but any specific colors or something?"
"I don't know.. I was never in the Underground. This is my first time meeting you." He sighed, hoping for a different answer. "alright, well we better start looking. we'll probably know it when we see it." You nodded along. "Hopefully." Day one was unsuccessful. As was day two. And day three. And day six. And day twelve. We found the first soul the third week in. "this is it.. this is what we're looking for."
"Frisk said there was six of these.." The soul was cyan in color and you picked it up. It was warm in your hands. "we need a place to contain them until we have them all." He took your hand in his. You both blushed. "and i know just the place."

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The soul was dropped off in his dad's lab. Day after day we went out and searched. Day after day Sans and I grew closer. I met his friends and his brother who also aided in our searches. By the end of the third month, we found four souls. We only had two left. "what're ya gonna do when we fix the timeline?"
"I'm going to find you.. and apologize to Frisk for not believing them before now."
"are you still a kid in the other timeline?" You drooped. "Yeah.." He shrugged. "that's to bad. i'm gonna have to wait for ya." You were confused. "Wait for what?"
"wait to date ya, of course! pretty sure there's a rule against it." You no longer trusted your voice and nodded, blushing a harsh red.

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You were on the search for the last soul now. After this, the timeline could go back to normal and you could talk to Frisk and find Sans. In your spare time, he'd explained what a soul mate was and that you were his, which is why he was gonna have to wait for you to get older. You were ok with waiting to get older for him. No dating before, you wanted to be the loyal one guy kind of girl. "it's orange.." Sans spotted it first. "Ok! We have them all!" You picked up the soul. "What now?" He shrugged. "beats me." He took your hand and you teleported to the other souls. He looked at his hands. "What?"
"i have an idea. i've been practicing my magic. i hope this works." Two rings appeared in his hands. They turned blue and the souls were absorbed. "Sans!" The rings turned yellow, sweat beaded his skull that was alot of power he was holding. You reached over and kissed his cheek. He squeaked, releasing the energy. Then everything went black.

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You woke up in your bed. In your timeline. You found Frisk on the way to school. "FRISK!" They whipped around, startled. "[Y/N]?"
"I met them! I met him!"
"I don't understand-"
"Sans, Frisk, Sans! I met him! He's a great guy, skeleton.. thing.. anyway! So I ended up in this timeline in the past..." You told them everything about your own adventure. About the souls, about Sans, NOT the relationship, about coming back home. "And I just want to say sorry for not believing you before." You concluded breathlessly. "Think I can help you free him? Free them?" After a minute of shocked silence, they nodded. "I need all the help I can get!"
"Let's free the monsters!"

~Fin~