Something nudged you awake. You slowly blinked your eyes open. Beverly was sitting in the sand next to you, looking radiant.
"That was fun," she said with a smile, "but it looks like you're back to your old self."
You sat up and examined your body. Sure enough, you were back in your normal human male body.
"Why do you think you turned into a mermaid again?" Beverly wondered. "We never got in the water."
"It seems like just been really aroused is enough to cause me to change." You gave a resigned shrug. "Sensuous creatures, or something like that. Magic is apparently not only real, but kind of strange."
The sun was clearly growing lower in the sky. "We should go." Both of you brushed the sand off your bodies (you tried not to watch Beverly too close as she did so lest it start your change again), put your bathing suits back on, and headed back down the beach. Your beach towels were where you had left them, but most other beachgoers were now gone.
"Maybe if we went to the carnival again I could get some answers about what is going on with me. Find out if it's permanent. If so, whether it's controllable at all."
Beverly agreed. After stopping by both your residences for a little further cleanup and change of clothes (you dared not shower) the two of you headed to the carnival. Your directed your girlfriend down the streets you'd walked the day before, although you made some wrong turns on the way. Finally you reached some familiar steel gates. Heading inside and follow the faint music to the carnival. Once again the carnival seems fully operational yet deserted.
"This place gives me the creeps" said Beverly. She took your hand and held it tight.
The layout didn't quite seem the same as yesterday, or maybe your memory was just fuzzy. The two of you wandered around a bit as you tried to find the right building. At last: there it was! You led Beverly to the door. The place still seemed deserted.
"Hello?" you called out. "Is anyone else here?"
Faint music. Rustling leaves. Little else.
"I guess we should look inside since we're here" suggested Beverly.
The two of you went inside. Your hands parted as you both scanned the many mirrors in the room. Shorter and taller, thinner and fatter. Monkeys, cats, werewolves, centaurs... Where was the mirror with the mermaid and merman in it?
You heard a short, sharp cry of surprise and turned to see Beverly standing in front of a mirror, her arm outstretched. The mirror seemed to show an underwater scene. You rushed to your girlfriend.
"What happened?"
Beverly didn't speak at first. She seemed dazed. "I...saw...in the mirror..." she stammered. "I knew I shouldn't touch it, but...I couldn't resist..."
You hugged your girlfriend tightly to comfort her.
"Let's go home" you said and led her, still a little dazed, out of the room. You couldn't help but glance back at the mirror. There, in a pose mirroring your own, was the mermaid you'd seen the day before: dark hair, large breasts, orange tail. The mermaid that seemingly now lived inside of you as a result of touching the mirror. What mer form lived inside your girlfriend as a result of her touching the mirror?