So Frisk at one point narrates "a hundred resets" and I would like to just warn you now: that is purely an exaggeration. It may have take this Frisk quite a few resets (and at least one True Reset, which they regret) to get the happy ending everyone deserves, but it didn't take them a hundred.
Frisk looked at Sans strangely, their usually laid back friend was sitting bolt upright, his legs jittering in place like he had more energy than he knew what to do with. Everyone in the car was a bit jittery with excitement, of course, they had only been on the surface for such a short while and still hadn't had time to go visit the beach even though the city was right next to it. Everyone was happy to make the trip once Frisk suggested it, but no one seemed half as excited as Sans if his bouncing knee was anything to go by. Frisk decided to ask him what was going on. They poked his arm.
"huh? oh, hey bucko, 'sup?"
Frisk looked down at where Sans's leg was still bouncing, then back up at him with a raised brow.
"i seem nervous? heh, just a bit nervous about seeing the ocean. can you blame me? the surface is huge and so different from the underground." he turned to look out the window, the next sentence murmured so quietly Frisk almost didn't hear it, "and it's been so long…"
"I d-don't blame him," Alphys said from Frisk's other side. "I'm a b-bit n-nervous myself."
"Don't be, it's gonna be a blast!" Undyne yelled gleefully as she leaned over the back of Frisk's seat.
"Seat belt," Toriel called in a sing-song voice from the driver's seat.
"DON'T WORRY, THE GREAT PAPYRUS IS ENSURING EVERYONE IS PROPERLY WEARING ALL APPROPRIATE SAFETY GEAR. ONE MUST ALWAYS TAKE SAFETY VERY SERIOUSLY." Papyrus leaned forward in his own seat, "I MUST ADMIT, THESE SEAT BELTS HAVE QUITE THE RANGE."
"Well… if everyone is still strapped in," Toriel said in a wavering voice. She seemed to be glaring at them through the rear view mirror.
"AND SINCE WE ARE ADMITTING THINGS, PERHAPS IT WILL COMFORT MY LAZY BROTHER AND NERVOUS FRIEND IF I ADMIT TO BEING JUST A TINY BIT NERVOUS AS WELL."
"Well I'm EXCITED!" Undyne yelled happily.
"OH, I'M EXCITED AS WELL! MORE EXCITED THAN NERVOUS EVEN, I CAN'T WAIT TO TRY THIS SEA BATHING THAT IS ALL THE RAGE. BUT! IT IS STILL! SOMETHING NEW! AND NEW THINGS CAN MAKE EVEN THE GREATEST A BIT NERVOUS."
Undyne nodded sagely.
"hate to sink your hopes, bro, but i gotta remind you that bones don't float."
"WHAT? BUT I'VE BEEN INFORMED THAT THE SALT CONTENT OF OCEAN WATER MAKES IT DENSER THAN THE WATER WE HAD UNDERGROUND!"
"not dense enough, bro. sorry, you'll have to stick to the shallows. though what's shallow for you and what's shallow for me are gonna be two different things." Sans leaned back with his hands behind his head and winked at Frisk, though his still bouncing knee wasn't fooling them one bit.
"Yeah, you're so short!" Undyne crowed happily. "But it's not like you'll even try, you'll probably lay down on the beach and go right to sleep. I can't WAIT to get in the ocean, Surface water has been so amazing so far!"
Sans looked at Undyne out of the corner of his eye and gave a soft huff, like he didn't believe her.
"DON'T FRET, BROTHER. WE CAN STILL ENJOY THE OCEAN EVEN IF WE CANNOT PLUMB ITS DEPTHS. OH! WE CAN BUILD SANDCASTLES! I HEAR THEY'RE A LOT LIKE BUILDING SNOWMEN."
"we could try building sandmen."
"THAT SOUNDS LIKE AN INTERESTING CHALLENGE."
"We're here!" Toriel said cheerfully as the van came to a stop. Everyone eagerly unbuckled and grabbed their bags as they all tried to get out at once. Frisk and Sans wisely chose to let the more energetic of their friends out first before climbing out themselves.
Frisk hopped out and stretched, it felt nice after the drive even if it really wasn't all that long. They made sure to be looking at Sans as he stepped out of the van and looked at the ocean for the first time. He froze, the little lights in his eye holes shrank to the tiniest pinpricks before blowing up huge. He started trembling hard enough for Frisk to hear his bones rattling before he gave out this strangled kind of wheeze, then took off running.
Frisk was startled, they had never seen Sans run before ever. Not in a hundred resets, not once since getting to the Surface. Apparently neither had anyone else because everyone simply stared after him in confusion.
Papyrus was the first to recover, "SANS?"
Sans was already out of the parking lot, having vaulted over the low stone wall between the cars and the beach (rather than go through the wide gap meant for people to go through), and was running over the sand. Frisk couldn't see his hoodie anywhere, and they were also pretty sure he'd lost both his sandals. Without missing a step he pulled his shirt off and dropped it behind him, then faltered as he tried to pull his shorts off without stopping.
"SANS!" Papyrus dropped the things he'd been holding, there was a loud crack so something likely broke, before running full tilt after his brother.
Like it was some sort of cue Undyne ran after him, Alphys hurrying behind at a much slower rate. Frisk started to chase as well, but Toriel scooped them up.
"My child, where are you going?"
Frisk frantically pointed at the others, looking back just in time to see Sans hit the water, then dive right into an oncoming wave. He didn't come back up. Frisk looked back up at Toriel with wide, frantic eyes.
"Frisk, my dear, there is not much either of us can do to help…" Toriel looked down at Frisk in concern.
"EVEN IF YOU CAN'T HELP DRAG THE GOOF FROM THE WATER, YOUR HEALING TALENTS MAY BE NEEDED."
"Oh!" Toriel and Frisk both looked to their side, where Mettaton had snuck up on them. "Mettaton, when did you arrive?"
"JUST NOW." Mettaton motioned to where his sparkling, brand new luxury car was parked next to Toriel's rental van. "DON'T WORRY, DARLINGS, I'LL GUARD YOUR THINGS. YOU TWO GO MAKE SURE YOUR SILLY FRIEND IS ALRIGHT."
"Thank you," Toriel said before striding quickly towards the beach.
As Frisk watched, Undyne was choking and coughing as she dragged a struggling Papyrus back out of the water. By the time they arrived at the small group Undyne had graduated to just making grossed out faces while holding Papyrus back, Alphys desperately clung to him from the other side.
"UNDYNE! LET ME GO! SANS NEEDS HELP!"
Undyne coughed and made another face.
"I-i-it's too late," Alphys said with a squeak.
A human wearing a bright red swim suit and carrying a red… thing walked up to them. A lifeguard, Frisk realized. "Is everyone okay?"
"I do not believe so," Toriel said as she approached. "Our friend ran into the ocean and has not come back up yet, correct?" She turned to the others for confirmation. Undyne and Alphys both nodded while Papyrus simply kept struggling. Toriel set Frisk down and gently moved Alphys aside so she could lay a gentle, restraining hand on Papyrus's shoulder.
"Can… your friend not swim?" The lifeguard asked nervously.
"H-he's made of b-bone," Alphys said with a gesture towards Papyrus. "He said that bone is uh… that bone doesn't float."
"Oh dear," the lifeguard said faintly. Then she looked away, down, up, then put a hand to the back of her neck and rubbed it nervously. "Okay, so I don't know much about monsters but can he uh… breathe? Underwater? Or uh… do skeletons even need to breathe?"
Everyone looked to Papyrus, who started to shake his head, then stopped and tilted it to the side thoughtfully. "WE BOTH NEED TO BREATHE, AND I DEFINITELY CAN'T BREATHE UNDERWATER, BUT… I'M NOT SO SURE ABOUT SANS."
"I am confused, would he not have the same breathing limitations as you?"
"THERE WERE A FEW TIMES I CAUGHT HIM FALLEN ASLEEP IN THE BATHTUB, OR THERE WAS THE TIME WE WERE WALKING THROUGH WATERFALL AND HE SLIPPED OFF A BRIDGE. THE WATER WAS TOO DEEP AND HE HAD TO CLIMB OUT ON THE SHORE, HE COULD HAVE BEEN HOLDING HIS BREATH BUT…" Papyrus trailed off, unsure. "BUT NO MATTER HOW YOU LOOK AT IT, THE WAY HE WAS ACTING TODAY WAS JUST WEIRD!"
"Well I'll keep an eye out for him, in the meantime please be careful and play in the ocean, responsibly, okay? And have fun!" The lifeguard gave a nervous smile and a little wave before returning to her post.
"Hey, before you go, what's wrong with your ocean?" Undyne asked to the lifeguard's retreating back.
"What do you mean?"
"It tastes awful! And I can't breathe it?"
"Oh, that's normal. Well, the bad taste is normal for humans, and we can't breathe any water so I'm not so sure about that. Honestly, all the monsters I've met so far love the ocean! There was this big tentacle monster that came by the first week after you guys… uh… the first week. Friendly guy, told me how much room he finally had."
"Ugh, whatever, thanks anyway."
"Alright, uh… good luck." The lifeguard waved again, then left for real this time.
"How are you feeling, Papyrus?" Toriel asked gently.
"CONFUSED, CONCERNED, AND A LITTLE ITCHY."
Frisk figured that last one was from the sand, or possibly the salt water slowly drying on him.
"That is all very understandable. For now let us trust Sans, very likely he is fine and will return shortly. In the mean time why do we not try to enjoy ourselves a little?"
Papyrus's shoulders slumped, "ALRIGHT." The whole group trudged back to the van to gather up all their beach things, then go set them up in a nice little spot on the sand. While the others were blowing up floaties and laying out towels, Toriel was rubbing sunscreen all over Frisk, followed by a life vest. Frisk pouted at that.
"I know you do not like this device, but I was assured this would be enough to keep you safe. I was also informed that the ocean is very dangerous for inexperienced humans, I suppose it is very dangerous for inexperienced monsters as well. And we are all very inexperienced, so forgive me if I am a little overzealous this first time here."
Frisk nodded, they understood that everything was very new to their family. They grabbed their kickboard and ran for the ocean, they wanted to surf! And maybe they hoped they would spot Sans out there. Eventually the others came out to join them, though they all mostly stayed in the shallows. Undyne didn't want to try breathing ocean water again, and Papyrus and Alphys didn't know how to swim. Frisk, of course, went over to play with them.
The day went on. Papyrus kept looking out to sea with a worried crease to his forehead, Undyne kept complaining about the water, Alphys got tired after a swimming lesson and went to join Toriel and Mettaton on the towels under their big beach umbrella. Frisk decided to go back out and ride the waves a little more.
"pssst, hey kid."
Frisk stopped and looked to their side. Sans was not that far away, though only his head from the nose hole up were above water. Frisk was so happy to see him again they completely forgot their kickboard to swim over to him.
"well it's good to see you too. and hey, you're not too bad at that, this thing seems like a bit of a nuisance though." Skeletal hands came up to support Frisk, though something about them felt weird.
Frisk didn't care, they were too happy to see Sans again and threw their arms over his head.
"oh, a hug too, today must be my birthday."
Frisk gently bapped him on the head, then pointed at Papyrus and Undyne.
Sans sighed, "yeah, i know. sorry, didn't mean to just run off like that. i can't get too close to shore right now though, go tell them i'm back, kay?" Sans angled Frisk towards their friends and gave them a gentle nudge.
Frisk turned and pouted at Sans, why couldn't he just go back on his own?"
"oh uh… guess you lost your swimming foam board thing."
Frisk shook their head, the vest would be enough and the board was probably already on the beach. They decided the easiest and fastest way to take care of this was just to go get the others. They started swimming, letting the vest support them as the waves pushed them back to shore.
"Hey there ya little sea squirt!" Undyne greeted enthusiastically. "Having fun in the waves?"
"GREETINGS AGAIN, FRISK! YOU ARE QUITE ADEPT AT SWIMMING."
Frisk swam over and grabbed onto Undyne with one hand, then held the other up.
"You want up? What? You tired already?" Undyne laughed as she picked Frisk up. Frisk just pointed out towards Sans. "What? Is something… Sans?"
"SANS!" Papyrus took off running, which was very awkward in water that was already lapping at his hip joint. He got out to where the waves were up to his chest when he had to finally give in. "SANS! CAN YOU COME CLOSER? I CAN'T GO ANY FATHER!"
Sans's head went underwater.
"SANS!"
His head came back up much closer to them, the gentle motion of the waves pushing him a little closer in spurts. Undyne had followed Papyrus out as far as she could while still holding Frisk, just barely close enough for them to hear the next part.
"heeeeeeeeeey bro."
"SANS! COME HERE RIGHT NOW!"
Sans slowly moved closer. "so uh… sorry about running off like that. i kinda lost my head there for a bit."
"LOST YOUR… SANS! YOU JUMPED INTO THE OCEAN AND DISAPPEARED! NOW COME ALL THE WAY OVER HERE."
"i can explain, but i don't know if the beach is the best place for it." Sans moved closer. Close enough for Papyrus to grab and lift him up, because that's just what he did. Papyrus was holding Sans under his arms, lifting him as high as he could. Now that most of his upper body was out of the water Frisk could see a pair of fins on the side of his head, they looked a lot like Undyne's but the spines were cyan and the delicate flesh stretched between them was bright yellow. And see through. There was a faint cyan glow between his ribs as well, and what looked like more cyan flesh underneath those. When the waves went low Frisk swore they saw more yellow fins.
"SANS… WHAT?"
"Why do you have FINS?!"
Sans held his hands up in a shrug, behind him something blue and yellow came very close to breaking the water's surface. "toldya i could explain."
"WELL YOU CAN EXPLAIN IT TO EVERYONE AT ONCE, IT'S THE LEAST YOU CAN DO AFTER THAT SCARE YOU GAVE US ALL."
"eh, fair enough. it's just i lost my shorts somewhere and uh… well… listen. just get me a towel or something and i'll come out of the water."
"NONSENSE, NO ONE CARES ABOUT YOUR ILIAC CRESTS." Papyrus turned and started walking out of the water.
"no bro, wait, that's not… eh whatever. it'll give us more privacy i suppose."
"What do you mean by that?" Undyne asked suspiciously.
"you'll see," Sans said as Papyrus marched past them. He threw a wink their way as well.
As the water got shallower, Papyrus shifted Sans to under one of his arms so he'd be easier to carry. Undyne and Frisk trailed a bit behind them, mostly so they could stare at the tail floating behind them. There was a fin along Sans's back, cyan and yellow again, instead of legs he had a long, cyan, kinda see through tail, and on the end was a large, bright yellow tail fin. Once Papyrus was out of the water and walking over the sand to the others, the tail started dragging through the sand. Sans must have not liked that because he shifted so the tail wrapped around Papyrus.
There was a loud, shrill tweet. A whistle, likely a lifeguard.
"Everyone out of the water! Siren!"
Yup, there was the lifeguard. Frisk wondered what took her so long to notice Sans. There were a few screams and quite a few humans scrambling to gather up their things and get off the beach. The lifeguard came running up to them.
"Sir! Sir I need you to put the siren down!" The lifeguard grabbed the arm that wasn't holding Sans (thought it was holding up his tail instead) and started tugging. "Sir! Let go sir!"
"WHAT IN THE WORLD? WHAT ARE YOU DOING? I'M NOT LETTING GO, WE FINALLY GOT SANS BACK."
"Sir for your own safety! please let go and move away from the siren!"
"WHAT ARE YOU TALKING ABOUT? SIREN? I DON'T HEAR ANY SIRENS. SANS, HAVE YOU BROKEN SOME LAW?"
"technically i'm not supposed to be on the beach, not sure if it's a law though."
"WHAT? ARE SKELETONS NOT ALLOWED ON THE BEACH?"
"That's stupid!" Undyne barked angrily.
"Sir! Let go!" The lifeguard tugged harder.
"Is there a problem?" Toriel asked as she walked over to the group, Alphys and Mettaton not far behind her.
Frisk squirmed, Undyne politely let them down, they ran over to the lifeguard and gently pushed her away from Papyrus. Once Frisk was sure she was looking at them, they mimed pulling something out of their ears.
"I'm not taking out my earplugs! That's a siren!" The lifeguard said loudly while pointing at Sans. "Sir! Please let it go! For your own safety! And the safety of everyone on the beach!"
"I don't understand," Undyne said in confusion. "Aren't sirens the loud things on police cars?"
Frisk made the "remove earplugs" motion again.
"WELL WHATEVER IS GOING ON, I'M SURE IT'S ALL SOME MISUNDERSTANDING. THIS IS MY BROTHER."
"Sir! Please! Let! Go!" More lifeguards were running towards them, the rest of the beach seemed to have emptied out by now.
"toldya it'd be easier with a towel."
"SANS! NOW IS NOT THE TIME! NOW! IF THIS FINE… HUMAN CAN'T OR WON'T LISTEN TO US PERHAPS WE SHOULD JUST LEAVE."
"That may be for the best," Toriel agreed. Everyone turned and started walking towards their things, Papyrus dragging along the lifeguard that refused to let go or stop asking him to let Sans go.
The other two lifeguards made it over and practically tackled Papyrus.
"HEY!"
The next thing Frisk knew they were being handed from Toriel to Alphys (when had they even been picked up?) while Undyne and Mettaton carefully pulled the lifeguards off Papyrus and Sans. Toriel joined the fray and it was quickly over, with Toriel and Undyne standing between Papyrus (and everyone else) and the lifeguards while Mettaton hovered protectively over Papyrus.
Undyne made the "remove earplugs" motion, to which all three lifeguards shook their heads and pointed at Sans again.
"That is a siren! You're in danger!"
"That is my dear friend Sans, who has been nothing but kind to me and my child."
"Yeah, and he's way too lazy to ever hurt anyone even if he did want to. I don't see why they think he's dangerous just because he suddenly sprouted a weird fish tail."
"it's 'cause i am a siren and uh… we are kinda dangerous. to humans."
Everyone froze and slowly turned their heads to look at Sans.
"what? it's not like i've ever personally hurt a human."
"W-wait, I've heard of sirens" Alphys exclaimed. "They're uh… a human myth, right? Half human, half fish, their songs lure sailors to their deaths and they um… they devour them."
"i'm not half human!"
"YES, HE'S CLEARLY HALF SKELETON!"
"and i wouldn't EAT a human! but yeah, the rest of it's right."
"OMG!" Alphys squealed and nearly dropped Frisk in her excitement. "I mean, I've seen them in a few anime but I didn't think they were real!"
"there's lots of monsters in the ocean still, not like humans could lock us under a mountain."
"Wait, wait, wait, you were alive for the war?" Undyne asked incredulously."
"nah, not that old. but i was born in the ocean."
"Okay, I took the damn earplugs out, can one of you tell me why there is a siren on my beach?!"
"THERE MUST BE SOME MISTAKE, THIS IS MY BROTHER! THE MONSTER WHO WENT MISSING EARLIER."
"no mistake, i'm really a siren, bro. and this is getting pretty uncomfortable, mind setting me down?"
"OH, OF COURSE!" Papyrus carefully set Sans down.
The lifeguard was pinching the bridge of her nose. "So your missing friend is actually a siren, who you pulled up onto my beach full of human victims?"
"i'm not gonna sing, geez." Sans rolled his eyes.
"You promise?" The lifeguard asked skeptically.
"i promise that i will not sing on your beach today."
"That was strangely specific," Undyne said.
"Alright, alright, but I want you gone."
"yeah, yeah, whatever."
The lifeguard turned and left, the other two trailing behind as they pulled out their earplugs.
"That was rather rude," Toriel noted.
"YES, BUT AT LEAST THAT'S TAKEN CARE OF." Papyrus picked Sans back up, this time carrying him princess style. They all walked over to their beach towels, where Sans was set down in the shade. He rolled onto his side and got comfortable, Frisk finally got a look at his hands, which had more bright yellow webbing between the fingers, and the yellow fins on his front. "ALRIGHT SANS, START EXPLAINING." Everyone else settled down around Sans, Frisk absently noted that the once overcrowded beach now had a wide empty spot around them.
"okay, first things first we're going all the way back to the war. not ALL of monsterkind got sealed in the mountain, obviously everyone still in the ocean was just fine. don't know about anyone else though."
"Cowards," Undyne said angrily.
"eh, i wasn't there, i'm not gonna judge them."
Frisk snorted and hid their laughter behind their hands.
"anyway, i was born in the ocean, obviously. when i was a little guppy, around frisk's age i guess, me and a bunch of other kids all swam upriver on a dare. ugh, fresh water, it's so gross."
Undyne snorted, "Like the ocean isn't."
"not to us, you freshie."
"What's THAT supposed to mean?!"
"just that you're a freshwater monster, you're not gonna like saltwater and i'm not gonna like freshwater. no big deal."
"NO WONDER YOU HATE TAKING SHOWERS OR BATHING."
"Why do we not just let Sans tell his story?"
"thank you, toriel. anyway, it turned into a kind of endurance contest: who would turn back first? who could go the farthest? i won, and the prize was falling into the garbage dump." Everyone winced at that. "yeah, you think the ocean is gross? the dump is downright disgusting. anyway, i got outta there asap, put on my legs…"
"I WAS WONDERING ABOUT THAT."
"oh yeah, the tail's our natural form, but we can transform at well. or normally we can, i've been wandering around on legs for so long that it's kinda messing with me right now, i have to wait to dry out." Sans wriggled his tail as if to prove his point.
"That's so f-fascinating!" Alphys said eagerly. "Do you mind letting me do some magic scans on you some time? Oh! And I have to hear you sing!"
"sure, sure, later. i did promise afterall. anyway, back to the story. got on legs and wandered around trying to figure out where i was and how to get home. that's when papyrus's dad found me and decided to take me in. and well… the rest you can figure out on your own."
"And you do not wish to return to your home in the ocean?" Toriel asked.
"well sure, for a visit. i was just a kid when i fell in the underground, i grew up here. you guys are my family now. though i wouldn't argue with picking up some salt on the way back so i can take proper saltwater baths regularly."
"I THINK THAT CAN BE ARRANGED."
"and hey, when we do get a house of our own up here why not something close to the ocean? something a bit secluded if we can, i gotta introduce you to the rest of my family."
"OH! I THINK THAT'S A SPLENDID IDEA!" Papyrus grinned, Frisk supposed he must have already decided Sans's birth family was also his family. Frisk could understand that, only Toriel may have adopted them but everyone here was family too.
Stuff that I couldn't cram into this: MTT shimmying into a full body suit (latex?) to keep the sand out of his joints. The fact that Sans started singing the moment he was underwater and his family is actually on their way to the beach. What other sirens look like. The giant salt water aquarium the skelebros buy for Sans to sleep in. Actually, it might be fun to some day do a follow up, both of the main cast meeting Sans's birth family and also what their home is like (I'm personally partial to a Disney's Little Mermaid style fantasy castle).
Bonus:
Sans: *the last note slowly fades*
Papyrus: WOW
Alphys: Oh. My. GOD.
Undyne: You sounded like a cat with its tail caught under a rocking chair.
Sans: hey, not my fault monsters can't hear the enchantment.