Dipper hid in the attic all day, and Ford stayed in the basement experimenting. Mabel made the excuses at dinner, saying that Dipper was feeling sick. But she didn't like lying to Stan, and was afraid that he could tell she wasn't telling the truth.
After dinner, Mabel grabbed a couple extra blankets, a few water bottles, and some food to bring up to the attic for Dipper. He had improved his coordination thanks to his endless pacing, but he still could not get onto his bed. Mabel put down the blankets in a sort of nest next to the bed that Dipper could comfortably lie in.
"You should eat something, Dip." She said, pushing the plate of food towards him.
He looked at the hamburger with a nauseated look on his face. He picked off the lettuce and tomatoes and left the rest.
"Hmm." Mabel grabbed a water bottle and sat on her bed. "Dipper, are you okay?" She took the lid off the bottle and took a few gulps.
He shot her a look. "What do you think? I'm a freaking deer, I can't tell anyone, I haven't seen anybody all day, and I'm starving!" He crossed his arms and turned away from her.
"Why don't you eat the burger then?"
Dipper shuddered. "I don't think I can. Like, maybe it's the deer thing but it's just… gross. And leave some water for me!" He said, looking at her. "Yeesh, are you going to drink all four bottles?"
Mabel realized that she had finished off the first two bottles already. "Oh, sorry. I'll be right back." She went back downstairs and grabbed more water, plus all the vegetables she could find.
Dipper sighed when she entered the room again. "Much better," he said, grabbing a carrot.
Mabel uncapped another bottle and drank half of it in one breath.
"You're thirsty." Dipper remarked, peeling off more lettuce from the head and biting into the leaves.
"I know," Mabel said. "Dunno why. Maybe I'm dehydrated." She took another gulp of water.
At that moment, a walkie-talkie buzzed in Dipper's backpack. "Dipper? Mabel? Are you there?" came Ford's voice through the static.
Mabel ran over and picked up the communicator. "We're here, what's up?" she asked, puzzled.
"I have reason to believe that even contact with the water from the Commutatus Fluvius may cause contamination," Ford said urgently. "Has anyone else touched the water?"
Dipper and Mabel looked at each other. "I helped Dipper out of the river…" Mabel said hesitantly.
"Is there anyone else?" Ford asked.
"I don't think so," Mabel answered, but Dipper gasped and snatched the walkie-talkie away from her.
"Ford, didn't you pick up my wet clothes? You touched the water," he said, sounding concerned.
There was static. Then, "Yes, I touched it."
"Ford, are you okay?" Dipper asked urgently.
"I'm-" Static interrupted him for a moment. "…I'll be fine. Radio me if anything happens to Mabel." The line shut off.
The twins looked at each other, scared. "What does he think might happen?" Mabel said, scared.
"I don't know." Dipper looked worried too. "On the bright side, if the same thing happens to you, we'll be twins again!" He gave a weak smile.
"I don't want to be a cervitaur," she said quietly.
Dipper felt a little offended, but he understood. "Trust me, neither did I."
Mabel snuggled up next to him in the blankets, using his long body as a pillow. She absentmindedly took another drink of water. Dipper wrapped the blankets around them both and waited for sleep to come.
But Mabel couldn't fall asleep. Lying against Dipper was warm and comfortable, but as the night dragged on she got thirstier. She couldn't understand it, it seemed like the more water she drank the drier she felt. Eventually she was so uncomfortable that she poured an entire bottle on her face.
Dipper awoke with a start, the cold water having splashed him. He rubbed one eye. "What's up, Mabel?" he asked.
"So thirsty…" she gasped. It was all she could say. She reached for another bottle, and Dipper handed it to her, looking worried.
"You look really dry," he said. "Like, your skin is flaking." He rubbed her arm as she guzzled more water.
"Oww…" She said, curling up in a ball under the blanket.
"What's wrong?" Dipper asked quickly. "Did that hurt? I-"
"My legs," she coughed. "My legs hurt." She gave a sudden gasp, and stopped moving.
"Mabel!" Dipper shouted, standing up quickly and stumbling over his hooves. He pulled her out from under the blanket and saw a tail. Not a short deer tail, but a long shimmery pink fish tail.
He gasped and almost fell over. She was a mermaid! She shook his head. He could think about this later, but right now he had to get her to water. He heaved her limp body onto his back and cantered into the bathroom. He started the tub and flung her into it, plugging the drain quickly. The bathtub filled quickly, and Mabel gasped and opened her eyes.
"Dipper? What happened? I-" she looked down, and saw the long tail snaking out from under her now soaking sweater. "Woah!"
"Yeah. So um, I think touching the river water works too." Dipper said with a small smile.
"No kidding." Mabel said breathlessly.
Dipper looked at her tail in wonder. "Weird that we turned into different things." He thought. "Do you think Ford changed too? Is that why he radioed us earlier?"
"Must be. I wonder what he is," Mabel said, still mesmerized by her shimmering tail. Then she yawned and stretched. "Maybe I can finally get to sleep. I feel much better now." She turned off the water as the tub reached maximum capacity, and curled into a ball at the bottom of the tub. It was deep enough that she could stay completely underwater if she didn't move too much. Dipper watched her fall asleep, tiny bubbles coming out her nose at regular intervals.
It was strange to watch her sleep so peacefully under the water, when he knew that yesterday he would have been performing CPR on her at this point. She'll be fine, he thought, and walked back into their attic bedroom. He was curling back up in his blankets when he remembered Ford's request. He picked up the walkie-talkie and turned it on.
"Ford? Are you there? Come in, Ford."
A roar came through the static in reply. Dipper jumped and dropped the walkie-talkie. He had felt that roar vibrate the floor, and heard it outside the radio. It wasn't as loud as it had been through the walkie-talkie, but his heart was still racing.
"Ford? Are you okay? What happened?"
All Dipper could hear was a low growling now, as if there was a lion on the other end.
"Um… F-ford?" he said slowly, his voice shaking.
The line cut off again. Dipper put the walkie-talkie down, his hands shaking.
It took him a long time to get back to sleep.