Already feeling strange sensations rippling over your body, you spun around and scrambled back the way you came. Your short brown hair was creeping down around your view, turning a dark purple while your facial features shifted to that of a beautiful woman. The rest of you was losing muscle mass along with any sign of body hair.
Behind you, one of the mermaids shouted, "No, wait! Don't go back through the waterfall!" Two of the others even threw themselves into the water to swim towards you, but they were too late.
Now barely keeping upright as your legs tried to stick together, you burst back through the water. On the other side, the woman at the podium gasped at your arrival. "Wha-Sir-uh, Miss, what are you doing!? You can't go through the waterfall twice before finishing your first transition!"
Falling to your knees, you clutched your midsection as it caved in, giving you a thin waist that contrasted nicely with your widening hips. Your legs finally touched and merged into a single appendage. As the merging reached your crotch, your trunks split and crawled up to cover your narrow chest instead.
Meanwhile, your manhood shrank away into a female slit before your thighs completely became one. Any sign of normal human bone or musculature vanished, leaving a graceful tail covered in shimmering purple scales. At its end, a caudal fin spread wide, showing off two black stripes toward the end.
"I don't even want a f-first transition! Please, change me back!" You whimpered as your voice jumped to a soprano mid-sentence. Suddenly, you gripped your chest as two breasts pushed out, reaching a good, eye-catching size under your swimming top. "N-No!"
By now, the woman had rushed to your side, putting her arm around you for support. A second later, a group of girls that you recognized as the mermaids from before emerged from a path to the side of the waterfall. Not only did they now have human legs, but their bikini tops now had matching bottoms as well.
"Looks like it's too late," said the tall one who had been the long mermaid.
Sure enough, the sensations had stopped. You were now completely a mermaid.
"What happened?" The receptionist asked.
The girls exchanged glances. "I guess she didn't read the paperwork," the Goth replied.
The woman's eyes widened. "Oh. Ah, that's a problem, especially for her."
"What?" You demanded, looking at their worried faces nervously. "What's the problem?"
"Well, you see..."