Sonja is just walking around the snow until something came out of the bushes which it startled her and fall into the freezing cold water. It's a Snowdrake that startled her which he gasped to see Sonja's Mermaid tail. Sonja's skin is all blue, her nose is red and she lets out a sneeze. She crawls out the water and used her heating powers to dry her tail. Sonja has her legs back and stands up slowly, shivering.

"How did you do that?" asked Snowdrake.

"M-M-My powers." Sonja coughs.

"Are you okay you look ill." Sonja rubs Snowdrake's head.

"I'm fine. I have to get going." Sonja told as she walks. 

She meets up with Sans again which he's talking to Papyrus.

"SO AS I WAS SAYING ABOUT UNDYNE..." Sonja tilted her head in confusion before letting out a sneeze. Her nose his getting red and white fish scales started to form on her skin.

"SANS OH MY GOD! IS THAT A HUMAN?!" Papyrus asked excitedly. 

"nah, I think that's a rock." Sonja crosses her arms in annoyance.

"OH." Sonja rolls her eyes.

"hey, what's that in front of the rock." Papyrus turns around and gasps.

"OH MY GOD! IS THAT A HUMAN?!" Sans sighs.

"yes..." Papyrus starts jumping in joy.

"OH MY GOD! I'M GOING TO...I'M BE SO POPULAR, POPULAR, POPULAR!" Papyrus shouted. Sans notices about Sonja's skin which it has white scales and her skin is pale. Sonja waves her hand back and forth, telling Sans that she's fine.

"AHEM, HUMAN YOU SHOULD NOT PASS THIS AREA! I, THE GREAT PAPYRUS WILL STOP YOU! I WILL CAPTURE YOU! YOU'LL BE DELIVERED TO THE CAPITAL! THEN...THEN...I'M NOT SURE WHAT HAPPENS NEXT...IN ANY CASE...CONTINUE IF YOU DARE, HUMAN! NYEH HEH HEH!" Papyrus laughed as he runs of. 

"well, that went well." Sonja smiled before letting out a cough.

"kid, you don't look so good." Sans kneels down and feels her forehead which it's burning up.

"you're burning up." Sans notices scales on her neck and white fish scales.

"I'm fine..." Sonja told.

"no you're not! look at yourself!" Sans gives Sonja and mirror which she gasped and screams loudly.

"OH NO! I have Mermaid Fever!" Sonja feels her skin which it's really scaley and slimy.

"What am I going to do?! I can't go like this!" Sonja coughs as she collapses on Sans' lap.

"don't worry, I'll help you..." Sonja looks up at him weakly.

"You will?" Sans holds onto Sonja's hand.

"yes...come to think about it...you're no ordinary human are you?" Sonja's eyes went wide.

"W-W-Well, you see...I'm a...Mermaid..." Sans gasps in shock.

"y-y-you're a-a mermaid?!" Sonja nodded.

"oh my god, please forgive my mistake! I thought you're a human and I have no idea that yo-" Sonja smiled.

"A lot of monsters made that mistake while I was at the Ruins. But I can transform into a Mermaid when I touch water and back into a human when dry." Sonja told.

"wow...you're really something else...it makes me really fishy." Sonja giggled at the pun.

"I'll tell go tell papyrus that he's making a mistake and cancel the plan to capture you." Sonja smiled before shivering again. Sans takes off his jacket and drapes it over her shoulders to keep her warm.

"hang in there, okay?" Sonja nodded gripping on the jacket, watching Sans teleport.

(Here's Chapter 5. I hope you like it and I don't own anything.)